Actually, there's a lot of skepticism around this.
First, theres allegations that they took code from the ODF plugin project, without credit, and relicenced it under the BSD licence. The original author isn't really pissed, and the original licence was essentially dual: Apache and LGPL, so it isn't that big a deal, but it wasn't proper if that's what they did.
Second, there is no damn reason for this plugin project. The new office project is a long ways away from being released. MS could put it in by default. Why don't they? Why make it a separate download? Free software or not?
Third, if this is a plugin, will it mean that people will have to make an extra step every single time they want to save in ODF? Will it say "you are saving in a sucky format and you won't get all the wonderful benefits of MS. Sell your soul Yes/No?"?
Fourth, this will ONLY work with their new, unreleased version of office, so people will have to upgrade to use it, while the current Free software version from ODF foundation works for everything from Word 97 onward.
Maybe I'm being too cynical, but this is MS we are talking about here.