I had the same problem as Macloyal in that I could not print on 4 x 6 photo paper borderless on my new Canon i6600. After searching around on other topics, I discovered that one must change the paper size in File>Page Setup to 4 x 6 and then choose File>Print (and choose all the other options here) to print on the 4 x 6 paper. You will know it will work because when you choose the Advanced button on the print selection, and then choose the Paper Handling option, you will have the 4 x 6 paper already selected as the default. Otherwise the US letter is specified as the default, and choosing the Scale to Fit Paper option and then specifying the 4 x 6 paper here does not work.
Hope this helps.