So, I typed "When will season 6 be on iTunes?" into Google, and, yes, the first hit I got was Doctor Who-related. Though I did get a bunch of other hits related to other shows' season 6s.
I didn't actually get a definitive answer to my question, but it seems that most shows are up a day or two after airing. I'm thinking that I might try renting the episodes through iTunes this time 'round rather than resorting to alternative means. I may prove too impatient*, but it's worth a shot.
*Or too incapable of avoiding spoilers on twitter/LJ, which is what I'm doing today