I love Linux and run it on many computers at home, and have never been particularly fond of Microsoft, but you really have to give this to them. It's more open than what Adobe have to offer and they're being far more cooperative than Adobe generally were. Remember, mono/moonlight are *open source*.. not even just a binary blob provided by Microsoft.
They've repeatedly helped the Mono developers and have truly made an effort to set a new precedent with many of their .NET technologies. In general, they're far more open (in specification and implementation) than their previous development efforts. Their new file formats aren't perfect, but they're certainly better than what came before.
I suppose that this, codeplex and their other efforts could be all a big plot to win mindshare from OSS developers and users and convert them to MS, then they'll break compatibility again, but really if they think that will succeed, they're living in a fairy tale world. Honestly, I do think this is an attempt to be a bit more cooperative, even for business reasons. The best thing we can do as Linux users is support actions that facilitate open platform and do our best to stifle technologies that are clearly an attempt to do the opposite. For example, if you work in a MS shop, try to get them to use their more open products over their closed ones.
What I think this is truly indicating is that OSS (and Apple) are hurting Microsoft's profit, and they're doing their best to hold their position in a more cross-platform market.