I'd do it in a heartbeat. If you have access to a MacBook Pro at work for "real work" (and you're thinking of a MacBook Air this fall), why not? The portability, battery life and to do the things you mention (minus the programming, although is that a total lost cause? I don't know, I'm asking).
Do it.
You won't get any more money down the road for your two-year-old iMac, especially once these newer models hit this year.
There are days where I wish a) I didn't do what I did, or b) that there was an iPad version of Illustrator (or something damn near close enough) because I'd probably chuck the Mac. At least for a while, in the name of "noble experiment".