You could buy a VGA cable for the PS3 too, though.
I'm fairly sure that the PS3 doesn't natively support VGA, and that the only VGA cables for the PS3 include a cheap converter. User reviews of the VGA cables I've found don't paint a pretty picture for these converter cables (although users reviews do tend to have a negative slant).
A large range of output resolutions through VGA straight from the 360 (using its very nice scaler) is great to have. You can use the VGA input for the 360 and DVI for the PC. I know I'm in a tiny minority of VGA users though.