Hope you guys realize the stars pretty much had to align for Apple to bring out the SB update the way they did. If you guys want to reference history, do so. Refreshes have been about a year for the last few major revisions. March/April 2012 isn't a stretch.
Apple was the first large manufacturer to bring Sandy to us consumers because of their agreement with Intel. When has Apple ever been early to the hardware party? Almost never. Heck, the MBP 13s were still running >2 year old Core 2s. MBP 15/17 was also late to Nehalem. We were EARLY to Sandy, which made the 10 month refresh possible (technically closer to 11 months, but who's counting?).
Furthermore, hardware changes. There WILL be new Sandy Bridge processors soon (SB E, at least on the desktop side), but quite frankly, I don't see Apple's thermal limits being able to handle such an upgrade if it were to affect mobile processors. On the graphic side of things, there are no new GPUs at least until early 2012. Apple "can" technically go for a higher end graphics card, maybe an 6850 or 6870, but quite frankly, I don't see that happening either. Apple manufacturers for mainstream, and where mainstream ends is ~6770. That's the highest card with a reasonable TDP (similar to a 6750), the "enthusiast" cards are all consuming some 60-70+ w, twice that of current cards. Would Apple utterly wreck their battery life for FPS gains? Unlikely. Furthermore, I don't think MBPs can support the thermal output of a more powerful card anyway.
So what's left? RAM bump, HDD bump. Possible. Is it worth waiting for? Not really.
What about a case redesign? Thinner you say? Problems. Thinner means even worse heat management than what we have now, and the current MBPs aren't exactly champs at diffusing heat. A smaller chassis means less air flow; this was fine with the MBA an its ULV processor and lack of discrete card, but MBPs are "professional" machines (albeit nowadays it's just a moniker), making it thinner in lue of getting rid of the graphics card and quad core processor? Makes little sense.