@vesuvius: I've seen people get bit because they had an unlimited plan, but they didn't have tethering. Logically it doesn't seem to make any difference, but the phone companies don't see it that way. They mean unlimited phone data, and if you want tethering, then it's an extra fee.
If I had an unlimited data plan I wouldn't be so annoyed about it since I could understand why they'd do it - in that situation you're likely to burn you way through far more data on a tethered PC than you would on your phone. That said, I think it's still wrong. Data is data.