The MacBook Air's US$1300 upgrade to a slightly faster processor and SSD, has just been benchmarked by a MacRumors reader at the only place they could get their hands on it, an Apple Store. By running Xbench on the 1.8GHz SSD model and compiling the results with the same test ran on the 1.6GHz model, the difference in speed is not too surprising. The obvious increase in the .2GHz processor bump is noticeable, but the biggest difference is with the SSD.
The most notable increase was in a Random Uncached Read test, where the HDD read at 13.28 MBs and the SSD at 48.24 MBs. Even though this is a substantial difference, it's expected since the HDD's head reads over various parts of the platter, where as the SSD has no moving parts. To see all the benchmark info check MacRumors for a compiled table. [MacRumors]