Username or EmailPassword
On the other hand IE9 will be available on Vista, thanks to exactly those technology backports from Windows 7. So I am not sure where this thread of arguments are going.
What we do know though is that IE9 is more or less a showcase of a lot of the newer Windows features, and as such probably exercise a lot of code paths earlier seldom used, so a baseline patch-level requirement for the OS is probably nothing to be surprised by.
As an aside; In many ways IE9 has really been a huge success for Microsoft even if it fails to get any users at all. All IE really does for Microsoft is that it ensures that Microsoft has a hand in making sure that Windows remains a top-tier platform for browsing the web. Adding good GPU acceleration (a key feature developed in the Vista/7 timeframe) nicely showcases Windows in that respect. Firefox adding Direct2D and Chrome being in the initial stages of doing so is really the best possible outcome possible for Microsoft, basically other vendors helping to add value to recent versions of Windows. Luckily it is certainly good news for consumers as well, plus other platforms get some of the advantages as well, making things better for everyone everywhere. Competition and all that