Liz, could you keep your G10 for the 'wide angle' stuff for now and the new camera for the 'telephoto' stuff? If so, it would seem your options for the 60D would be the 70-200 f/4 L IS or the 70-300 L. Either lens is not a light weight lens by any means, but they aren't crazy heavy either. My 70-200 F/4 L IS is a little heavy at times, but I don't mind because it gives me great sharpness, color, and contrast. Your idea about renting a lens is a good one as well. If you are wanting to shoot things in the wild with a 60D, 200mm probably won't be long enough.
If you can't keep your G10 for 'wide angle' stuff, then I would suggest renting two lenses
I just don't know of one lens that will 'do it all'. My 18-135 IS is NOT what I would call a sharp lens. It's fairly good at f8-f11, but other than that not so much.
On a separate note, I would love to try one of the G series cameras