According to a couple of alleged Best Buy inventory system screenshots obtained by 9to5Mac, Apple could refresh its MacBook and MacBook Pro range on Friday, March 11. At least that's when Best Buy is expecting to have them on store shelves, which means you should probably expect a formal announcement from Apple a little earlier.
There's little indication thus far of what the update would entail but Sandy Bridge processors seem like a given. Apple went on record during its Q1 earnings call that it viewed the MacBook Air as the "future" of the MacBook line, thanks to its slim form factor and almost instant-on capabilities. It's a reasonable possibility that new MacBooks could feature flash memory and an aesthetic makeover. Axing the optical drive to make more room for battery has also been suggested, a notion that doesn't seem too far-fetched given Apple's success with the Mac App store for digital software distribution.
For now this is based on no more than speculation and conjectures, but with inventories reportedly running low on MacBook Pros at official Apple retailers, all seems to indicate that at least some kind of update is on the way. We should also point out that Intel is officially resuming shipments of their Sandy Bridge chipsets to certain companies, with fixed parts arriving in mid-February, so maybe there won't be much of a delay for Apple's MacBook update.