Spent a few minutes last night trying out an external flash on the G10. I used it with the Canon ST-E2, triggering a 580EX speedlight, firing into a custom built light tent (or a big cardboard box with panels cut out of the side). The shot of the G10 is taken with the 1DII with the 100mm macro lens. The shot of the EOS-1 was handheld with the G10 in macro mode. (note the slight difference in aspect ratio between the two cameras)
I was quite impressed with the control available on the G10, in all manual mode. You can switch the flash into full manual too and control the power output directly from the camera. You can also use the standard flash through the lens metering (E-TTL) and adjust the flash power up and down using flash exposure compensation. In this fairly easy lighting scenario, it did really well and produced a great quality image, even handheld. I've heard good things about using the G10 in a studio and it balances quite well with the ST-E2. Certainly better than putting a speedlight directly on top of the G10, which would really unbalance things. The 580EX is slightly bigger than the G10 itself.
For the G10 shot I held a sheet of paper up, just to the camera right of the shot to bounce some light back into the lens opening and add some more light. Small changes like that can make a surprising amount of difference. Overall it might be a subtle change but it can really make or break the feel of the shot.