We still need to figure out if this is a 7290-specific feature (there are several rumors from several sources saying that the 7290 has modem capability), or if this will work on older BlackBerries. This may be due to hardware support for serial-over-USB.
In that case, a custom third-party modem driver may still need to be written if older Blackberries do not support serial-over-USB operation, then routing must be done via a proprietary method as mentioned in this thread, which is something that a third party company may or may not bother doing. (At least, until everybody's switched to the 7290 and the complaining stops :D )
Time will tell before this is confirmed.