vCenter 2.5 Update 6 was released on Friday. Whilst I'm not working with any 3.5 / 2.5 environments at the moment this is good news because Windows Server 2008 R2 guest customisations have been added. Also added is support for Firefox 3.x using vCenter Web Access. The full release notes are here.
I'm going to stick my neck out a bit and suggest that this may be the final update to vCenter 2.5 before it reaches the end of General Support in May.