Initially I decided that I would just wait to buy the unlocked version next month. However, I then decided otherwise, and here's why:
1) Buying now "saves" me $200 right now. My total out of pocket after taxes is $595 for the 32 GB. Had I waited to buy full retail it would be about $811. I realized that over a year from now or whenever the next iteration is released, I really don't forsee an unlocked 4S netting me over $200 more than a non-unlocked one. In my experience, unlocked sells for between $100-150 more. I've found, however, the biggest price differential stems from when you sell your phone. Listing and selling BEFORE the official announcement is the key whether locked or unlocked.
2) The "unlocked" 4S isn't truly unlocked. You will not be able to sell it to a Verizon or Sprint subscriber. Stateside it would only be unlocked to AT&T and T-Mobile - since I won't sell to overseas buyers I won't consider other GSM carriers in this reasoning. Now, T-Mobile has all but said that they'll be getting an iPhone (whether the 4S or next version) in 2012, and assuming that pans out, I'm assuming T-Mobile users will be less willing to shell out huge dollars for a phone they can only use on EDGE when they will likely have the option to get the fully functional version.
3) I'm perfectly happy on AT&T, have great 3G speeds in the Dallas area, and don't anticipate needing the "flexibility" of being contract free as I would just stick with them anyway. As far as being "off-cycle" for next year's iPhone I'll just be in the same boat as now, no biggie (new contract price + $250 - I will likely get nearly this amount for the 4S by timing its resale next year properly).
4) The "unlocked" 4S may be another 6 weeks for all we know. I can't stand to use my original iPhone that much longer. Push notifications aren't working on it and I'm sick of having to check TextFree manually.
All of these considered there's really no huge benefit to buying the "unlocked" 4S.