Can I use my iPhone as a fax machine modem using the Bluetooth connection?
I tried to find a solution to do this about a week ago, and found that if you're in the US with a non-jailbroken phone, you just can't. 

Yeah, it annoyed me, too.

There are a number of different Bluetooth profiles; here's Apple's list of Supported Bluetooth profiles. No faxing on there, dammit.