1,1
I also have a 2006 Mac Pro 1,1 and have been thinking about the same things. Last week I installed Adobe Creative Suite 6 and there was some warning about my graphics processor not being "officially supported" -- I ignored it and everything seems to be working fine, but that can't bode well for the future. I also want to move to Mountain Lion when it comes out, for specific reasons I can't explain here -- trust me, it will make my life easier.
My thoughts are that Apple has got to know that there are a decent number of happy Mac Pro 1,1 owners who are going to be looking to upgrade for the same reasons -- current and near-future software is leaving them behind. So this is a major watershed -- the anchor for the transition to Intel will soon be obsolete.
Last week I went to buy a new machine, but what held me back was not the likelihood of a refresh right around the corner (WWDC?) -- these machines are plenty powerful for my needs -- what held me back was the less-good iSight camera in the current Cinema Display, as compared to the better HD FaceTime camera built into the new Thunderbolt Display (which doesn't work with the current Mac Pro). I use FaceTime and Skype a lot, so it felt like I would be buying out-of-date tech for that. Anyhow, it stopped me from reaching for my wallet.
So count me among those waiting for a pretty significant refresh, or even a redesign. Something that will work with the Thunderbolt Display, or a refreshed Cinema Display.