The first MacBook Pro came out on February 14 2006.
The unibody MBP came out on October 14 2008.
That means the original design lasted 973 days.
It's been 860 days since the unibody design.
Given that MBP refreshes typically occur around 8 months (243 days) apart, the next refresh (after thursday's one) would be around October 25 2011.
That's 1106 days after unibody's introduction.
If the new design were introduced this Thursday, the unibody design would have lasted 110 days less than the original one.
If it were introduced in another 8 months, it would have lasted 133 days more than the original design.
Following that logic, it's as probable to have a design refresh this time as to have one with the next refresh.
As much as I would like to see a completely new MBP right now, I see Apple changing everything (standard SSD, new slimmer casing, Light Peak, etc.) at once with a single update which would be worth an event with the press and everything. I know Apple doesn't do an event for every product redesign, but it's the MacBook Pro we're talking about here. Apple's top-selling computer. The MacBook Air, Mac Mini, iMac, Mac Pro, even combined, don't sell as much as the MacBook Pro.