For those keeping score, last time it was Windows Server 2008.
For various reasons (that I won't bother reiterating here, since there's no value in doing so), I wanted to make our next release 64-bit-only. There's a little bit of precedence in that Exchange 2007 had already done so, and the way we're currently thinking about upgrade recommendations (a post for a different time) would keep it from being painful in the majority of (but not all) situations.
Thankfully for our customers, that got shot down and our upcoming release is currently slated (plans change, nothing's nailed down, etc) to be supported on both 32-bit and 64-bit. More work and more testing for us, yes, but good news for customers, especially those that wanted to do in-place upgrades from Orcas (on an OS of 32-bit 2k3).