If you use an MBA with SSD and a beautiful display for a few days while on the couch, in an airplane, in bed, at a client's office, or etc, YOU WILL NEVER WANT TO LEAVE THE FORM FACTOR!
We can all say how practical the 13" MBP is now with the far superior capabilities of RAM... but with SSD, a 2.13 GHz CPU, and soon Snow Leopard, THE MBA IS EXTREMELY CAPABLE FOR THE AVERAGE MAC USER!
Connect it to a 24" LED ACD, and you have an instant desktop. Perfect docking station and even power connector... USB hub, camera, and SPEAKERS which the MBA really benefits from!
Remember too, that extra RAM doesn't really make the Mac faster. Extra RAM is ONLY used when you use many apps or highly intensive apps that need the RAM. Most people using a 13" display, aren't using the heavily intensive apps, but when hooked up to 24" ACD, YES.
So, RAM in the MBP is the big difference, along with battery, and MAYBE a better display. BUT, the fun in the MBA is unmatchable by the "practical" MBP 13" notebook!
I plan to use my new 2.13 GHz MBA with SSD as my primary Mac with a 24" LED ACD. I am a business consultant, lecturer (college), public speaker, and writer. For my needs, the MBA is incredible. I am a business "PRO," and I think the vast majority of users like me would far more enjoy the MBA. I don't believe that people will NEED the extra RAM offered by the MBP. BUT, the battery could be the reason for some to lug around the extra weight.
Nothing beats the MBA when it comes to fun. But as a work computer, portable ALL DAY LONG on one charge, with a beautiful display, perhaps fun isn't what I should be looking for??? But I use my MBA for everything, so it's used just as much for personal entertainment/computing as professional work. Whether I am on the couch, at a client's office, or giving a presentation with Keynote, I LOVE LOVE LOVE the MBA! But maybe my love should be for the Mac with more power, longer battery usage, and better display???
I am going to work on both for a few weeks and totally review them. Will report back.