I think it's fast enough for most people's everyday use of word processing, excel, internet, email, iphoto and everything that non-professionals use daily. If you're working with something that NEEDS more power, you'll already know that and not be looking at the macbook air. If you don't know that you need more power, you probably don't!
Not too sure what 'non-professionals' means here.
I consider myself a professional - I'm a technology consultant working with Fortune 500 companies - and I've switched over completely to my MBA as my primary system. I've made only one compromise in my day-to-day use - I wish I had one additional port. Could have been either USB or Ethernet and I'd be perfectly satisfied.
My old system was a two year old MBP15. One of the first gens. It had a 100GB drive and 2GB RAM.
I ran (and moved entirely) a WinXP VM running in Fusion over. I typically dedicate a Gig of RAM to the Win VM and use it quite a bit onsite with my current client. Some of the things I do require Visio, many require Project, and with my current client I use Outlook to access their email system.
I am not a casual user - I'm a technology architect, my current engagement involves re-engineering eighteen application ecosystems. I do light development (my job involves design more than code these days) and other than the reduction in screen real-estate, I don't see any drop-off in performance from the MBP. The Core2Duo (replacing my olde CoreDuo) certainly makes up the difference.
In my office I have a 20" cinema display and USB hub. I use a bluetooth mouse and keyboard, and operate using dual screens with the 20 as my primary when at my desk. I use wireless to connect (not an AirPort, it's a dlink WAP at the moment, 802.11G) and have not had any problems at all.
Have I made a few compromises? Sure, I carry the Superdrive if I think I'll need it, as I said before I'd add one more USB port if I had designed it (primarily for Ethernet use onsite with clients) and I lost some screen. I also lost about 3 pounds in weight and gained about 1.5 hours in batter life. Gained a nicer display as well (the old tech is really noticeable to me now).
Bottom line to me: while it's not for everyone as their primary system, it will work great for many, many people. Don't underestimate the performance or the broad range of use you can get from an MBA