This is my opinion but I think the 13 inch MBP will be the first to be updated as it is the oldest by design and a stronger market demand for some type of upgrade. There is speculation that this update will not use Sandy Bridge Quad Core but we have no idea and and for all we know they could release an 8 core MBP that runs off of geothermal energy and methane; people are going 'huh?', but think about it, Apple has released a lot of things out of the Blue which took the market by storm and surprise. Also Apple customers tend to wait on the next release as opposed to buying another brand and Apple knows this which gives them the luxury of spending longer on a design and that actually makes sense as you can make a better computer (in theory) if you have time to test it extensively, experiment with multiple production possibilities and aren't working to beat the clock at whatever costs.
The MBP releases have followed:
-Early 2006
-Late 2006
-Mid 2007
-Late 2007(sucks for the people who got Mid 2007)
-Early 2008 (really sucks for the people who got Mid 2007)
-Late 2008
-Early 2009
-Mid 2009
-Mid 2010
-??? 2011
The only thing you will be able to get out of these release dates is that they have absolutely no correlation with anything whatsoever. We can't say when it will come out, but I can tell you that Apple's official release sentiment is "whenever we feel like it for the lulz" (that was actually in an email from someone from Apple). So if you feel a strong need for a new computer, I'd buy one without too much worry about the upgrades as your last MBP has got you through for half a decade and the current ones are just as well made.
If you upgrade your existing computer, do stuff that you can use on a new one as you will obviously be buying one in the not too near distant future. A lot of people may say get a SSD and get one with a size that you will be happy with on your new MBP. I think SSD's are pretty awesome and that's a worthwhile upgrade as you can use a SSD in any SATA computer and they are really fast.
