They've already released Windows Live Messenger http://itunes.apple.com/us/app/windows-live-messenger/id3761...
as well as a Bing client. http://itunes.apple.com/us/app/bing/id345323231?mt=8
Vectorworks (used to be called MiniCAD) for Mac is still excelling in areas AutoCAD seems to have forgotten about. Its 2D drafting is unparalleled, although use in a multi-user environment is lacking. There is also ArchiCAD; a system second only to Bentley. Once one adapts to the different method of drafting in these apps they rarely want to go back to ACAD, if only for import/export checks.
Much like moving from Word to Pages... annoyingly odd at first but much more productive once learnt fully.
[EDIT] Considering that OneNote is part of Office, a major cash-maker for Microsoft.
All in all, meh. At first sight, it reminded me of Evernote, but I think it's missing what makes the desktop version of OneNote interesting: the ability to take notes however you want by just clicking somewhere. The size of the iPhone doesn't really allow for that so I can understand. But if you're not using the desktop version, it looks like this is no special note-taking app.
This article doesn't mention it at all, but it would be a great thing to see on a mobile (even if it included a cloud round-trip).