My 2c
Im going to chime in on this with my 2c. I have both the current top of the line MacBook Air 13 (1.8GHz i7, 4GB RAM, 256GB Flash) and an early 2011 MacBook Pro 13 (2.4GHz i5, 8GB RAM, 750GB 7200rpm HDD).
I had the MBP first, and picked up the MBA about a month ago because I wanted to reduce some weight as I am often traveling with a MacBook and an iPad. I reduced my on machine data down to about 115GB by moving iTunes and archiving out of use projects and documents to an external HDD. I've been using the MBA as my primary machine for about 3 weeks with some and/or all of Mail, iTunes, Safari, Firefox, Photoshop, Dreamweaver, Illustrator, eCub, iBooks Author, Pages, Word and Dreamweaver running.
I love the snappiness/respponsivnes of the MBA, especially the almost instant on/off and zero bounce application launches, unfortunately, when in full go mode with almost everything running I was regularly maxing out the RAM, and suffering performance issues. The processor never struggled for my workload.
Now, I've been running with at least 8GB on all my machines for the last few years, so I have never had this issue before. As a result, today I am going to pick up a Crucial M4 256GB SSD for the MBP (might upgrade the RAM to 16GB while I'm at it!) and sell off the MBA. I have no doubt if the Air had 8GB RAM it would be amazing for my needs, but, I am keen to see how the upgraded MBP operates. (May order the OptiBay kit and put the 750GB HDD internal and keep my barely used SuperDrive outside over USB.)