This has been referenced several times (see, e.g., the Groklaw article "Reactions to MS's "Glimmer of Openness" & a Little Water Under that Bridge") but you really need to read this piece by Andy Updegrove "The British Library, XML, and the Rest of the Story".
This sentence by Andy is particularly telling: "In short, the British Museum is not recommending the XML Reference Schema as an exclusive solution, but giving a sigh of relief that if Microsoft has decided to offer some sort of methodology to permit long-term preservation."
While you are at it, take a look at Steve O'Grady's piece "The Office Productivity Standard Wars Heat Up". Right at the end of this you should see why ECMA was chosen as the venue for the Office XML work: fast, minimal changes to input specs (so much for the community), and a safe haven of IPR.
The role of the British Library in the ECMA move