Well, I'm somewhat stumped. I figured out that its the Mobile-Stream USB Modem application (
http://mobile-stream.com/usbmodem_wm.html) that added the BT DUN profile, but I can't for the life of me figure out how to "extract" the profile from that application and get it to stick around. As soon as I uninstall the program, the PPC no longer responds as having the BT DUN profile. I did lots of registry comparisons, and can't find any unique keys that would have any control over whether or not the profile was there, so I believe it is a function of the DLL's loaded. The install file, USBModem.cab, contains only 2 files, 1 shortcut, 1 reg key, and the configuration. The files installed are "usbmodem.exe" (the program), and "usbcdcacmfn.dll" (library). The shortcut is "USB Modem.lnk", pointing to the usbmodem.exe. The reg key is HKLM\Drivers\USB\FunctionDrivers\USBCDCACM_Class, which adds a number of values that I do not believe are related to Bluetooth. As far as I can tell, the Bluetooth profile is created when you choose the "Enable" option after running the "USB Modem" program. It certainly does a lot to the system, as choosing "Enable" keeps the PPC busy for a little while.
What I *really* want is a PPC utility similar to the Windows REGMON and FILEMON utilities, that will monitor all file and registry accesses while I run the "USB Modem" application. That or "/usr/bin/strace", but Linux is much easier to work with.
I searched for a good system-call tracer or debugger for PocketPC, with no success. I have VS.net 2005 on my desktop, but it looks like you can only debug PPC apps if you insert debugging symbols into the application to start with. Since I didn't write the app, and am trying to reverse-engineer it, that's not gonna work.
Ok, that's all I've got to offer. Hopefully someone else will take that and run with it.
BTW, the "USB Modem" application is $19.95, and doesn't work to allow modem access over USB (lots of USB timeouts logged on my iBook). I was using the trial version, and I'm not going to pay for it since it doesn't work as its supposed to.