It's a tough one, the 24" iMac does look really nice. Like Chris says, don't underestimate the usefulness of a 24" screen for photo editing. I've got a 22" and it's lovely, obviously a 24" is better.
If you were going to play any recent ish games on it, I'd say the iMac is a bad idea. 1) because the screen is huge and therefore takes more processing power to fill it, and 2) because the integrated graphics on the cheaper iMac 24" is pretty poor. You get a much better graphics card on the Macbook Pro, and a smaller screen so games should whizz along nicely.
There are rumours that the next version of Photoshop will use the graphics card for some stuff, so in theory the MBP wins slightly there, but the higher CPU speed on the iMac might balance that out anyway... Either way that's a long way off and isn't for definite, so I wouldn't worry about it.
It's a tough one. Don't underestimate how useful it is to be portable though. Even if it's just for the extra desk space you have when you put away your computer.