I have a DroidX and I can tether using the built in stock Hotspot feature or with Bluetooth. However, I can't tether using an ad hoc connection because I get an error stating that the device doesn't support ad hoc connections...
So, it is definitely device specific and it will be something the carriers remove as many report being able to do so with root (my device is rooted and I can't connect via ad hoc - although, I haven't d/l or tried out the pdaNet app).
I will also add that the bluetooth tethering has worked MUCH better than the wifi - easier/faster to connect, runs pretty fast, and my phone doesn't overheat as much....
Galaxy Nexus & Asus Transformer (TF101)
ATT Samsung S - Captivate
Various Window OS devices
Always waiting (patiently) for the latest CyanogenMOD
Galaxy Nexus (LTE) - running AOKP ROM = Awesome!
Thanks to JesusFreke, Cyanogen, LucidRem, Stericson, Haykuro, The Dude, Wysie_Soh, Kmobs, Koush, Wes, Birdman, etc. - Ya'll are awesome!