I try to stick to mine for 2 years, or at least until upgrade time. Been 100 percent successful up to now. With the elimination of the annual early upgrade and the NE2 discount, it's gonna be much harder these next two years. Aactually it's still 20 months on a 2 year contract, not a full 2 years. Especially with all the new hardware coming out.
To answer your question, specifically:
A lot of people do pay retail for early upgrades. Then they sell their old phone on ebay.
VZW has a trade-in program and will buy back your phone for a not-so-hot price:
[HERE] Right now the TBolt is worth $240 for trade-in.
Also (this is me) a lot of people have family share plans of 2 to 5 lines. VZW allows you to shift upgrade dates around to different handsets on your account. In fact, you can even do this on line, automatically. It's easy. So, if you have two lines that are staggered, you'll get an upgrade each year. 5 lines and you'll get one more often. Of course this assumes that whomever in your family is holding that phone will not get that upgrade, 'cuz you're steeling it!
Also, I believe that due to the cutting back by VZW on the perks I mentioned above, you will be seeing more "private offers" and such (i.e. Sir, you as a preferred customer have been selected to receive an upgrade of your choice before the end of your contract", etc.). This way VZW can still maintain tight control on who gets early upgrades.
Also, also

I would be remiss if I didn't mention that some of us are old farts who have worked all of our lives (well, not yet!

) for the ability to buy these overpriced toys whenever we want! Hahaha...
-Frank