Teensy adds less height if mounted the other way up, but it's still about 7.5mm from the bottom of the main PCB to the peaks of the solder joints on the Teensy. I used an IC socket, and solid core wire for the pins (much thinner than the pins you'd get from PJRC, but not too thin).
Obviously care has to be taken so the USB socket doesn't short out on the solder joints of the switch above it, but actually it ended up even closer to the PCB than shown in this picture, after the switch's pins were trimmed back a bit...
[ Attachment Invalid Or Does Not Exist ] 32716[/ATTACH]
Since it's in a socket, it's removable for reflashing etc