Interesting thread with interesting speculation. I jump over to the NS forum every once in a while to see what the latest is with the first of the Gingerbread phones (I'm looking forward to getting GB on the Droid X, but it may be a while).
The only upcoming phone I'm looking at is the dual-core Bionic (Also on Verizon). That plus the higher resolution display and 4G-LTE data is what I would consider a real upgrade - no word yet as to the OS, but I would think at least Froyo, possibly Gingerbread.
To the OP, any phone is pretty much "obsolete" the day you buy it, but that doesn't mean it won't serve you well for the length of your contract (1 or 2 years, or longer). I did some research before I bought my DX back in September, knowing the EOL was in 6 months I got it and couldn't be happier with my purchase. I'm sure the Nexus S will serve its owners well long into the future after its no longer a "premier" phone - there seems to be lots of people still happy with the original N1 (and original Droid). I think its important to do your due diligence research-wise and make the plunge - lots of variety and options out there, and competition is a good thing