I took your advice and bought the 58mm polarizing lens thru the net and there was VIGNETTING!!!!!!!!!!!!! i bought the Hoya ones!!!!!!!!!!!!!!!
I'm sorry to hear your having that problem. Are you using the same set up as I am. My lens is the WD-58 .7 made for the Canon GL1 video camera it isn't marketed as a lens for the powershot digital cameras although it works well (i think it is a little more expensive). Maybe there is a difference between that one and the one canon pushes? Canon has the WC-DC58N listed as the accessory wide adapter for the G3 on there site. I'm not useing that one. I'm also using a lensmate 58mm adapter so no step up rings. I didn't use a hoya filter I just used a genaric no name one I've had for the past ten years bought at "blacks" a canadian photo finishing store. But it doesn't look like a special thin filter, looks as wide as any other filter I've seen.
All I can tell you is that my set up allows me to use my entire zoom range with out vignetting. Let us know if your using a different lens so we can all learn what the magic combo is or if I'm just lucky.