Microsoft has officially announced Windows Mobile 6.1 - and it looks like a number of devices will be able to get it as an update in addition to the newer devices coming out this year already having it. You can see tons of screen shots at Gizmodo.
While I was already planning to update the ROM for my Tilt (a known/reported start button issue - it's occasionally unresponsive), now I may just wait and see if I get the 6.1 upgrade at the same time.