To answer your a)... Because most users don't think about their browser and hence use whatever the default is. OEMs will likely never bundle Firefox, but there is a chance that they'll bundle Chrome since it has Google's name behind it.

Hence, Chrome rescues users (developers?) from IE.

As for your b)... I agree totally. Chrome is nothing but an application delivery platform for Google apps that is disguised as a browser. Which is all the more motivation for Google to pursue bundling deals with OEMs.

Why would Chrome save users from Windows when they already have Ubuntu as an unbundled Windows alternative?

If the Google name gets people who currently use IE to try out something else, that's a win for web developers.

Chrome is different than the other browsers in its slimmed-down interface, sure. But as other browsers, and the computers they run on, get faster and more compliant, the advantage that Chrome has over them as a client for web applications will erode. Apparently FFX 3.1 Beta 2 already has a faster js engine than Chrome's beta did.

Yes, it is. At first, I thought that maybe it's because Linux hackers weren't stepping up to help with the development efforts, but then I started wondering how many Windows hackers helped out with the Windows version.