For everyone unsure about the speed of SSDs, you need to know that they definitely very depending on Manufacturer and model, just like regular HDDs. There are many different components and even flash types that affect the speed of an SSD.
With the Macbook Air's SSD, Apple probably had to compromise on speed for cost, weight, and probably most important, power consumption. It's understandable, but I am still disappointed by the performance somewhat. (especially for $999... a 32GB drive should be $500 or less)
If I were to purchase an Air, I'd definitely get the HDD, and swap in a SSD myself. For the $999, you can get a faster SSD, and for a little more money, a 64GB SSD.
There have been many recent announcements from different companies about very fast upcoming SSDs. I just read the other week that Samsung has a new line of SATAII SSDs with claimed sequential read and write speeds above ~100MB/s including in 1.8" SSD drives.
Here are some test results from tomshardware.com on some SSDs they have reviewed. Now when you compare these to the Macbook Air sequential read/write speeds, you'll see that the Air test results have a breakdown for block size. I am not sure how the tomshardware.com results are calculated, e.g., if they are an average of block sizes, or what not. But it should give you a good idea of their respective performance.
MTron SSD 32GB
95 MB/s sequential read
75 MB/s sequential write
Sandisk SSD6000 32GB
68MB/s sequential read
47MB/s sequential write
Samsung FlashSSD 64GB
55MB/s sequential read
30MB/s sequential Write
RiData TurboSSD 32GB
55MB/sec sequential read
26MB/sec sequential write
http://www.tomshardware.com/2007/12/17/solid_state_drives/page7.html