The Christmas special will probably be included with season 6. That's the convention they've followed with previous Christmas specials. That is, they include them with the next full season DVD release. They're usually available as separate purchases too (in the UK anyway).
The one thing that may be different this time around is that season 6 may end up getting broken up into multiple sets since the show is being broadcast in that manner. The first 7 episodes will be airing in the spring and then the final 6 episodes will be airing in the autumn. Whether the BBC breaks the season down into multiple box sets is hard to say at this point. But it wouldn't surprise me if they did to maximize profits.
Actually, the BBC gave a very upfront statement that, although they will release multiple box sets for s6, they will also release a 'normal' box set comprising the entire s6. So, if you want to save money, wait till all the eps are over. If you want to see the eps as soon as possible, you have multiple box sets waiting ;)
I'm still not sure that we Yanks are going to get the split box sets for series 6... Amazon.co.uk has had a listing for Series 6, Part 1 for quite a bit now and there's no trace of such a thing here on regular ol' Amazon.com.
I DID purchase the Christmas Special due to uncertainty of how the series' box sets would work now, but (as for the two halves) I'll definitely wait until the complete Series 6 box is out due to things like the the full series sets having DTS-HD HR 5.1 audio as opposed to the plain stereo sound they've been putting on the 'rushed' Blu-rays and DVDs (i.e. all of the single discs with 3 or so episodes that they've put out in the UK since 2005)