I had no issues with photoshop when I tried an Air for two weeks. I'd had preferred more RAM, but for most photoshop jobs 2GB suffices. Contrary to belief photoshop does not require an amazing amount of graphics power to run well, just RAM and processor. 2GB works well, unless you're running a lot of background apps with photoshop, which will still run okay just slower (obviously). The 1.6 Core 2 Duo did a very admirable job at most tasks, including photoshop.
Personally the bright screen was perfect for photo's, but the RAM limitation was an issue. If a MacBook Pro is too big consider a MacBook, which can be stuffed with a lot of RAM and has plenty of processor power and even a 7200 rpm drive can be added easily for that added bit of performance in big photoshop jobs.