I used powerbook 1.5ghz and i barely used 100% of my cpu power. I used my macbook 1.83 core duo, i rarely get to 50% of cpu usage. I used my laptop for web browsing, emails, microsoft office and IM.
So people who say MBA is slow should think about what kind of use they are planning. Unless you are opening a lot of programs (meaning more programs that i mention above, video editing, or adobe programs, which means you probably want a MBP) you wont need that much power. I am currently using powerbook (3 years old) and im happy with it. Its not the fastest, but it gets the job done without delaying my work.
For 1.83 core duo (intel chip thats one year and a half old is still more than enough. MBA 1.8 is faster than 1.83 core duo.
(all of this assuming you have 2gb ram) which is what the MBA has.
I will defend MBA cpu speed. It is fast enough for web browsing, emails, microsoft office and IM.
It is true that most people don't need much power, and the people that do probably shouldn't be looking at the Air to begin with. I was never considering the Air, personally, but that's because I need at least a 15" screen, more than 3GB RAM, and apparently more than the 2.33 CPU I currently have in my MBPro. Of course, I'm constantly running Adobe applications, Maya, Rhino, AutoCAD, and 3D Studio Max thorugh Boot Camp among other apps. It's true that the Air seems to be geared more toward the Business, Office/e.mail/internet user than users that require incredible performance. People definitely need to realize this when judging the Air, and not expect it to be a Macbook Pro.