I tweeted this information yesterday but it deserves a broader announcement here on the blog. The next full version update of Lightroom will not run on PowerPC-based Mac computers. Lightroom 2 updates will continue to support PowerPC.
The Creative Suite made the same announcement yesterday via their FAQ and John Nack added his color commentary as well.

I think Adobe is gone nuts.
Many photographers still use G4-G5, the latest is not older then 3 years.
So what must I do as photographer who bought LR appr 2 years ago.
Do I have to sell my G5Quadro? I'm NOT nuts.
So I start looking for an alternative whitout arrogance.
Cheers
Roeland
there is no mandatory requirement to upgrade to 3.0
You can keep using LR2.5 and future 2.x updates. Why on earth would you 'look for alternative' just because the latest software wont run on your hardware? You wont keep your G5 forever and when your ready to upgrade you will be ready to go to 3.0
Sorry, but your only disadvantaging yourself.
I have now personally moved to Intel based macs but know many photographers still running power PC based chipsets.
Mac hardware typically has a longer life than corresponding PC kit