(Note see various 2017, 2016, 2015 and 2014 postscripts at the end of this posting)
Update 24 October 2019 Thank you Filbert Lam for alerting me to the Singapore Court of Appeal reversing the High Court Update- I have no access to the CA judgment yet – reasons and analysis to follow.
Update 7 July 2019 for review of a recent Austrian SC decision adopting a favor validitas approach see here. For a view from Singapore see BNA v BNB  SGHC 142 reviewed here: the High Court interpreted an express provision for “arbitration in Shanghai” to be an agreement to Singapore-seated arbitration with hearings in Shanghai, thereby upholding the validity of the arbitration clause and the jurisdiction of the tribunal.
Update 15 May 2019 Whether Fiona Trust is good authority in Australia might have been, but ultimately was not considered in  HCA 13 Rinehart v Hancock Prospecting Pty Ltd. For review see here update 12 November 2019 and Michael Douglas case-note here. The High Court found it unnecessary to consider whether Fiona Trust is good law in Australia. According to the plurality (Kiefel CJ, Gageler, Nettle and Gordon JJ), the appeals could be resolved by application of orthodox principles of contract interpretation, without reference to Fiona Trust: para 18.
Preferring to settle issues by arbitration (often preceded by mediation) continues to be a preferred method of dispute settlement in commercial transactions. It is most probable that the best results in arbitration are reached for contracts of a sizeable value, between companies with pedigree, with a certain amount of contractual history between them. However even then, lack of attention to detail may land parties in a pickle. In Sulamerica, the claimant insurers seek the continuation of an interim anti-suit injunction against the defendant insureds. Parties are at loggerheads over the validity of an arbitration agreement between them, which may be found in the policy. Express choice of law for the policy has been made for Brazil. Express and exclusive choice of court has also been made for Brasil. Parties are all Brazilian (incidentally, the re-insurers were not). The subject matter of the insurance is located in Brazil (Jirau, one of the world’s largest hydro-electric facilities). However the arbitration agreement in the contract concludes with appointing London as the seat of the arbitration. Arbitration was agreed to be held under ARIAS rules.
(Not just) under English law [see the House of Lords in Fiona Trust], an arbitration agreement is treated distinct from the substantive agreement in which it is included, for the purpose of assessment of its validity, existence, and effectiveness. This leads one to have to ascertain
lex arbitri (the law of the arbitration agreement, per the preceding sentence);
the curial law or the ‘law of the seat’ (the procedural law which will guide the arbitration proceedings; despite the latin curia not commonly referred to as lex curia);
the ‘proper law’, the law that governs the actual contract (lex contractus); and
the locus arbitri and the lex loci arbitri: the venue of the arbitration and its laws, which may or may not interact with the proceedings. Update 8 January 2018 see for an example of such impact the new Chinese approach to optional arbitration proceedings, applicable as of 1 January 2018).
In the EU, the issue is not covered by the Rome I Regulation, for arbitration is excluded from that Regulation. Whence the courts apply their national conflict of laws rules. In England, this implies identifying the law with which the arbitration agreement has its ‘closest and most real connection’. In Sulamerica, Cooke J held that this was, in this case, England, given London having been assigned as the seat of arbitration. Indeed in Abuja International Hotels, Hamblen J came to the same conclusion with respect to an underlying agreement that was governed by Nigerian law.
The lesson here is clear. With three sets of applicable law having to be identified, one had better consider them specifically, in writing, in the agreement.
Postscript: Cooke J held in January 2012. In May 2012, the Court of Appeal confirmed the decision.
Postscript 2, 3 July 2014: In First Link Investments, the Singapore High Court took a radically different approach in May 2014, noting that “it cannot always be assumed that commercial parties want the same system of law to govern their relationship of performing the substantive obligations under the contract, and the quite separate (and often unhappy) relationship of resolving disputes” and that “the natural inference would instead be to the contrary”. (Case come to my attention thanks to Alistair Henderson and Daniel Waldek). Postscript 4, 2 December 2016. In BCY v BCZ the High Court would seem to have entirely altered that position, reverting back to Sulamerica.
Postscript 3, 2 June 2015:In Trust Risk Group SpA v AmTrust Europe Limited, the Court of Appeal further considered the House of Lords’ presumption of the one shop principle and decided it did not apply to the case at issue. The CA, upon detailed analysis of the agreements at stake, decided in effect that the later agreement was lex specialis vis-a-vis the overall business agreement between parties and hence that choice of law and choice of court of the later agreement prevailed. (Davina Given and Ed Holmes posted on the RPC blog with full review of the case). The Court’s analysis highlights among others the often less than clear language used in commercial agreements, whether or not caused by the fog of closing. In particular, the agreements under consideration used often confusing and not clearly defined concepts to denote the various agreements at stake.
Postscript 5, 25 October 2017: in Roger Shashoua v Mukesh Sharma CIVIL APPEAL NOS. 2841-2843 of 2017 the Indian Supreme Court once again had to emphasise the difference between venue (lex loci arbitri if you like; potentially only the place where hearings are held) and the seat of arbitration (which determines procedural issues; the lex curia). See review by Herbert Smith here.