Good news for those of us who still have PowerPC-powered Macs lying around: while the new Garageband Learn to Play feature isn't actually designed to work with the old machines (part of Apple's switch to the new Intel chips), it apparently still does. If you've got iLife installed on your old Mac and double-click on the Learn to Play files themselves (hidden in /Library/Application Support/GarageBand/Learn To Play/), Mac.Blorge says that they'll work just fine. Unfortunately, you won't be able to buy new lessons from the store (people are still testing -- there may be a workaround here eventually), but if you want to play the ones you've got, they should work, even if playback isn't perfect.
Additionally, if you want to try to do a little hex editing, you may be able to get iMovie '09 playing on a PowerPC Mac as well. That one's just dodging the PowerPC check, though, so there's a good chance that some things won't work right on the old machine. Either that, or Apple is just trying to build in random requirements to get us to upgrade. Conspiracy hats, anyone?
At any rate, this isn't unexpected -- we're two years past the official switch, and of course at some point Apple had to move on with their new software. For the moment, you might get things working with a few tweaks, but eventually you'll have to look at replacing that old G4 if you want to run the shiny stuff.