Apple may make great computers but some of their decisions are questionable - like not making Airport Extreme for G4 PowerMacs. The G4 is a perfectly capable computer even in today's marketplace, so are we all consigned to 11Mbit wireless?
No. If you buy a wireless PCI card that uses the same chipset (Broadcom 54G) as Airport, OS X will recognise the card. It took me a while to find one here in the UK (apparently some of the Buffalo cards have it, but they're more readily available in the states). So if any PowerMac users in the UK want to get fast wireless, look out for a BT Voyager 1040 wireless PCI card - you can get them in PC World for around £30. Pop it in and OS X will believe it has Airport Extreme.
Hope that's of use to someone out there.