USB:
- Digital Camera
- HDD Video Cameras
- External Hard Drives (slow..)
- Keyboard/Mouse
- Webcam
- Printers
- Scanners
FireWire:
- Digital Cameras
- MiniDV and a lot of consumer video cameras
- External Hard Drives (very fast..)
- Target Disk Mode
- Audio Interfaces (prosumer and consumer level)
- Video Interfaces (prosumer and consumer level)
Off the top of my head. I haven't included every possible device obviously. But saying that the things that are USB far outweighs the things that need FireWire is an ignorant claim.
Remember, Apple ships GarageBand, iDVD and iMovie with all Macs. These are pretty much useless for anyone planning to take these applications seriously.