When the iPhone 4G rolls out, are you going to wait till Summer 2011, or sell your 3GS on Ebay/Craigslist to offset the upgrade penalty? Odds are, the new one will be a substantial improvement over the 3GS, a complete upgrade, so patience might pay off, but I guess if you can sell your 3GS for a good price, there's really nothing wrong with constantly upgrading.
Offset route. I sold my 8GB 3G for £280 ~ $460 and used that towards a new 3G S. If i didn't get a nice chunk change i probably would've kept my 3G so yeah...as long as i get decent resale values i'll probably keep updating
EDIT: Truth be told if the Pre was available in the UK i probably would've gone for that instead. *Shrug*