I have a 2009 MacBook Air, bought a year ago - will I notice much of a performance improvement if I buy the latest version (Late 2010)?
The flash storage makes a huge difference (a fresh system boots in about 15 seconds, apps will start instantly etc.), and the new MBA does not need to throttle the CPU when it gets hot. So you will probably notice a big improvement. See Macworlds benchmark results, for example.