ArsTechnica provides an extensive walkthrough and installation of the OWC MacBook Air SSD replacement kits. The OWC Mercury Aura Pro Express is the only presently available SSD replacement for the MacBook Air's new compact SSD drive.
Installation requires some disassembly of your MacBook Air as well as the lengthy backup/restore process to migrate your data. The new drive offers some performance benefit over the original Toshiba SSDs, and is about on-par with the recently shipping Samsung SSD drives.
The main benefit for the replacements are increased storage capacity, but it comes at a price with 180GB/240GB/360GB drives costing $499.99/$579.99/$1179.99 respectively.