What you can't run on a Mactel
Ivan at creativebits has composed a list of application types that just won't work with Rosetta. From his list:
The Classic environment (and hence, all pre-OS X apps)
Screensavers written for PPC
Code that insert preferences into the Preferences Pane
Kernel extensions
Apps that depend on one (or more) PPC-only kernel extension
There are a few more, check them out. I said "See ya" to Classic a long time ago, so I'm not really concerned about the Mactel being the official nail in that coffin. As Ivan suggest, just look for the universal logo above when making purchases for your Mactel, just to play it safe.