According to Live Feed, AMC is planning on bring the show back next October. The idea is that AMC's horror marathon, FearFest, serves as the perfect promotional platform leading up to the show. It did definitely seem to work out just fine with this first season, which is pulling in some of the largest numbers AMC has ever seen.
I don't know how much the second season is going to need that promotion, though. The show has already shown to be so much more than the standard horror fare its teasers on AMC sold it as. I'd actually like to see some statistics on these serials, in terms of how many new viewers come on in successive seasons, as I suspect the people who loved this first season won't need any more convincing to watch the second.
The other reasoning being given, which seems to hold more water, is that AMC's dance card is pretty damn full for the year. Breaking Bad has been slated for the summer instead of March, and Mad Men will also be returning in its usual summer slot. AMC also has a new series, The Killing, which will also be premiering in 2011. Even with the cancellation of Rubicon, this is more original series than AMC has ever had to promote, and my impression is that they want to give each the promotion and support it needs to succeed.
So it seems that unless they could shoot the whole second season within the next couple months, I can't see a better spot in the schedule than next fall for The Walking Dead. It'll be PAINFUL to have to wait that long, especially after this short season, but that's not really the worst news in the world. Establishing the show as having a regular premiere slot around Halloween each year sounds like a pretty fitting position for it.