I have an iBook G4 I bought in late 2004. It was about the equivalent level of technology for its time as a current MBA, and it still works just fine, 3 and a half years later. Is it as "snappy" as my daughter's new MacBook? No, but it does what I need it to do. I can certainly surf the web, run itunes, etc.
Some people would find the computer's performance to be unacceptably slow, but for my needs, it's fine. It also doesn't hurt that I have a much newer iMac that I use for most of my work
3-4 years is about the life of a computer these days, and I think the MBA will be no exception. You can usually squeeze out another year or two after that, but then things tend to go downhill pretty quickly.
If you're really concerned about obsolescence, why not get the MBA with the standard HD instead of SSD? In three years, you'll be able to sell your old MBA, pool it with the money you saved from not buying an SSD today, and buy a brand-new 2011 MBA with a 512 GB SSD.