Already exists as an add-on, eg the GP-1
for Nikon; and of course many of the high-end DSLRs support a cable that will connect them with any 3rd party GPS that conforms to the NMEA 0183 spec.
I suppose it would be cool to have it built-in, but I don't know that I'd be willing to pay a lot extra for the feature. Also it may not work as well as it typically does with cellphones, because the latter can also use tower triangulation when conditions (tree cover, urban canyons, indoors) make GPS signals unavailable. Though I suppose we're not that far away from popping a SIM card into our SLRs as well, and uploading our pics as we take them (not to mention web browsing via the display on the back of the camera...)