I'm in love with my Canon G9. In many ways the Canon G10 is even better but until today I've not been able to find any way to put a wide-angle converter on the G10. I just don't consider the 28mm lens on the G10 wide enough for real estate photography.
Since www.lensmateonline.com has add-on products for the G9 and G10 I posed the question to the sales folks at lensmate.com. Here's what they say:
The only wide angle that works on the G10 is the Nikon WC75, we do not sell this lens, but we can make you a 58-52mm co-planer stepring to allow you to marry the Nikon to our Lensmate G10 Part A. "...to order the Co-planer, make a note in the message window "please add a custom co-planer 58-52mm step ring at $7.95".
Update on 3/24/2009: A reader commenting below says you can also use the Raynox SRW-58 0.66x converter along with lensmate part A described above to get 18.5 mm effective.