I've modified the board of the Teensy++ ver 1 and added a jumper to select between 5v and 3.3v. I spoke with Paul (the developer) and compared the schematic for the Teensy v2.0 board which has a place to install a 3.3v regulator and then cut the lines of the Teensy++ to do the same. See the notes here for details:
http://www.flickr.com/photos/deadhacker/4152517331/
Paul explains that ver 2 of the Teensy++ will have a location for this regulator. But for anyone with a ver 1 board... ftw.