It seems no one's really done any speed testing, but I found some info on another forum which theorized that misaligned blocks shouldn't affect the read speed in most cases (I'm dealing mostly with large video files anyway), but the write speed may be slower.
I didn't want to take a chance, so just ordered 2 of the previous generation Samsung HD203WI 2TB drives. Newegg is sold out of these but Provantage has them (at a good price).