USB Tethering not working?

[techluvr]

I use an iPhone 4 on iOS 5.0.1 and a late 2009 MBP on Lion. My USB tethering is not recognized at all on the computer. It used to work fine, but now it does not. I have a tethering plan from AT&T, so that isn't the issue. Repeated restarts of both devices have done absolutely nothing. WiFi tethering works fine- I'm typing this post with it, as a matter of fact. Any help appreciated. Thanks!