The "Strobist", David Hobby, has an interesting and useful post about using the G9 with Nikon flashes and high shutter speeds on a photojournalism assignment.
I regularly use my own G9 with the Nikon SB-24, SB-28 and the inexpensive (OK, cheap) Cactus wireless units. Here are some postings about G9 and external flash.
Compatibility List for G9 Flashes (please contribute your own recommendations)
Simple slave flash triggered by the G9
Flash attachments on the G9 external flash
Sync speed for G9 and non-Canon flash
LCD display with various flash setups
Using multiple Canon flashes with the G9