without the ability to calibrate the screen any form of aperture of photoshop will be nothing more then a gimmick.
Ive heard a lot of people wanting aperture for the ipad, I honestly cannot see the utility unless you just want to have it. The ipad is not a photo editor.
Who's to say calibration wouldn't be possible? After all, if Apple decided to make an iPad version of Aperture, it's quite possible they would make an app that could control/calibrate the screen.
Secondly, Photoshop can be used for a variety of things were perfect color calibration isn't necessary. While an iPad version of Photoshop could never match up with the "real thing" as long as the iPad is running the iPhone software, it could be a great supplemental product.
For instance, create a series of PSDs on a real computer, work on them on the iPad while you're out, while you wait at the doctor's office or the DMV, or the airport, etc. Then finalize them (if necessary) when you get back to your main computer or just send drafts to clients/teammates/etc. directly from the iPad.
Is it as good as having a laptop? No, but then the iPad isn't as good as a laptop when it comes to real computing. But that doesn't mean they can't make it more of a professionally-useful device.