Well, that was fast.
Looks like you can now download the ChevronWP7 custom ringtone manager--the first homebrew app for Windows Phone 7. The program is a .XAP (pronounced "zap") file and can be deployed using the Microsoft “Application Deployment” tool with the Windows Phone 7 Developer SDK.
To use the app, you basically build a custom .XAP file with up to 5 ringers on board and then "install" it to the phone (kind of like the .cab managers of Windows Mobile days). Ther ringers have to be in WMA format at 48KHz as "required by the OS", so there will be some encoding before you just copy over.
While you can't use the app on-the-go, it sure beats being limited to the default ringers. Remember, you need to unlock the phone first before you attempt and you are taking a risk, albeit small, here.
Source/Get it here: ChevronWP7