If you haven't, I'll try not to spoil anything for you. Suffice to say it was very inventive - a speculative steampunk delve into a "what if" scenario (featuring some favorite recurring baddies, to boot!)
My understanding is that the 2008 Christmas Special is the first of four specials that will take the place of a full series in 2009. Expect the 5th series to begin in 2010 with Steven Moffat at the helm instead of Russell Davies, and a new, younger Doctor - guess that means a regeneration sometime this year in one of the specials, nu?
Sorry, I don't know whether or not the BBC is going to release these 2008-2009 specials as individual DVDs or as a set at the end of '09.
One thing is for sure - waiting sooooooo long between new episodes of the Doctor has got me squirrely!
Yes, the 2008 Christmas special is "The Next Doctor." Officially, I think the 2009 specials are consider part of series 4 - if they're part of any series. I have read that with 2010 they will be calling it Series 1 because it's a new production team. Personally, I hate the idea. Just like the editor of Doctor Who Magazine once pointed out, there is no old or new series. It's all the same show. The 2005 series was a continuation of the original series, not a reboot like with Battlestar Galactica. There are references to the older episodes. It's the same Sarah Jane Smith that was with the 3rd and 4th Doctor. In the original run, they didn't change the series numbering with a new production team.
In case you don't already know, both specials have been released on DVD in the US. Since Planet of the Dead was the first HD episode, it's also available on Blu-Ray.