There's a GOOD chance the PS3 price would be cut down to $200 by October 2011 (just like the Slim price cut happened just in time for Uncharted 2), and I think that would benefit PS3 and the U3 sales mutually and very significantly.
I also think the game being in 3D will also play a major role for Sony 3DTVs and the PS3. I'm looking at a BIG investment on marketing and advertising because of this. By the end of 2011 3DTVs will be receiving a major push at retail, price tags would be down considerably and PS3-3DTV-Game-Glasses combos will be very common. Uncharted 3 is going to be a 3D killer app, definitely, and Sony is going to exploit that.
If the game delivers at least the expected AAA quality, I'd say it should easily exceed sales expectations. It's way too early to call, but considering all the factors I've mentioned, it should outsell Uncharted 2 sales with ample headspace.
@CGI I don't think there'll be any whispers about PS4 anytime next year, and even if there was, or the system were to be announced at E3 2012 or something (highly unlikely), I doubt that would play any role in diminishing Uncharted 3 sales. I consider a $200 PS3 having a quite bigger positive impact going for U3 next year and for 2012.