Iyaz Akhtar over at theappleblog points to a nifty little device from Twisted Melon called the Manta TR1, which enabled the Apple IR remote on any older Mac via USB. Just plug in the Manta to a free USB port and your old G4, etc. can rock with an Apple Remote, just like the newest Intel Macs. The Manta is meant to work with Twisted Melon's own Mira Apple Remote software, which we mentioned earlier. In addition, the Manta hardware supports IR blasting (to control other devices) but that functionality is not "used with Mira at this time." Check out Iyaz's original post for more info on RF Remotes.
The Manta TR1 sells for $19.99, and Mira runs $15.95, or you can get them bundled together for $32 from the Twisted Melon store. Of course you'll also need an Apple Remote if you don't have one already; they run $29 from Apple.