A Daywalker, is in fact a real drink created by the screenwriter. It consists of Jack Daniel's, Triple Sec, gin, ginger ale, and "just a little bit of grenadine."
One of the zombies in the nightclub is credited as Ke$ha. She is killed with a bottle of Jack Daniel's.
The concept developed from some of the producers talking about how it is to use Craigslist in order to assemble a zombie mob. Most of the extras did in fact respond to Craigslist postings, in full makeup and costume.
When the survivors are debating the technicalities of zombies, the Male Lead indicates that no one made a list of rules, a possible reference to Zombieland, in which the main character survives thanks to his own list of rules.
If you look closely, many of the zombies are also played by members of the cast and crew. As filming began to wrap, extras began to diminish. In fact, Erik Troyer Skoog ends up shooting himself from the roof before he jumps.