I got a chance to play with one today (iPad). They are amazing and super fast. A guy brought his in and didn't know how to use it, so I kind of showed him the ropes as the experienced mac dude who has been doing nothing but reading about the iPad. That thing screams. It is soooo quick. Safari, I tap it, and pops open as quick as on the macbook. Very thin, very light weight. I actually didn't mind the keyboard. The cute clicking noises it makes as you type on it (with it laying down is easiest) makes it feel much more familiar. I didn't notice a terrible decrease in my typing speed, still about 40wpm instead of the usual 50-60.
That said, get the iMac. They are AMAZING computers, and will serve you well for much longer. Also, as previously stated, the iPad will be perfected in second and third generation models. Remember how the iPod didn't get super cool until its current state, the touch. Imagine 6th gen iPads! AHHH! But yea, the point is, 500 bucks isn't a ton to save up after you spend 1200. Wait a year on the iPad, see what happens (like the front facing camera I only know they have to be putting in). In the meantime, enjoy a super sweet upgrade to your computing experience.