From my own experience with this, I bought a Samsung Moment a couple weeks after it launched and was abandoned with about 17 months left on my contract (with some severe bugs that were never resolved while I had it). I did get an official update to 2.1, but it was very late and that was the last update it ever received while I owned it.
Sure, I wasn't an eligible purchaser for their next immediate phone release, but their utterly terrible support of my product ensured that I'll never be a future customer of Samsung, for any of their product divisions and severely tarnished my impression of Android in general. All Android manufacturers do equally bad jobs at supporting their products with only a couple of products as exceptions (even "big release" phones like the original Motorola Droid got abandoned before their earliest contracts ended, late buyers are really screwed), so I'm not too keen on rolling the dice on any future Android products either.