NOT THAT IT MATTERS MUCH, BUT A FEW SPOILERS
It might well be that the zombie movie is all but exhausted as there's not a lot more that you can do with them. They've stumbled aimlessly about, they've shuffled remorselessly, they've run, they've climbed the walls. They've rebelled, they've been domesticated, they've used tools. But this is probably the first movie in which the Romero-descended screen zombie has talked - not just with its own kind (as in Wasting Away - there the living couldn't understand it any more than the dead could understand the living) but with ordinary humans - and has not wanted to eat anyone.
The Deadheads are Mike and Brent (Michael McKiddy and Ross Kidder): a couple of guys who wake up in the woods during what appears to be the zompocalypse: the dead are up and ripping people to pieces. But these two guys are dead - victims of a US military experimental something-or-other. They're still thinking and talking - and decomposing. All Mike wants to do is explain to his ex how he feels about her, and to tell her how and why he died, so the two of them and a tame genuine zombie named Cheese hit the road to find her. But the military is on their trail as well....
Horror comedy is a pretty difficult thing to pull off and the number of movies that have managed it is not high - the most famous is probably An American Werewolf In London. Many think they're making it as horror comedies by spoofing horror films (the Scary Movie series) or being ironic about them (the Scream series), but little or no nerdy injokery in Deadheads unless Brent's surname Guthrie is intended to reference Guthrie The Loonie in The Living Dead At Manchester Morgue, which is perhaps unlikely.
For all the jokes and bursts of zombie action, Deadheads doesn't make it as a horror comedy because it isn't really a horror movie: aside from the opening zombie attacks it's much more of a wisecracking romantic comedy/road movie that just happens to have undead characters in principal roles. And as a comedy it does score well: most of the characters are nice and normal (zombiedom notwithstanding) and you don't mind spending time with them, and the result is a charming and enjoyable little film. It's not Shaun of the Dead, and it's not Dance Of The Dead (which I really liked) but it's still nicely done and rather sweet: more "ahhhh" than "aaaargh".