Well that depends. Do you want to apply for the 2009 cycle? If you've just decided you want to go to dental school, I think a late DAT score isn't the only problem you'll have if you don't have any extracurriculars or anything like that to show that you're serious about dentistry and that you actually have a pretty good idea about what you're getting into.
Whether or not December is too late is dependent on a lot of things, especially how competitive your GPA is, and whether or not you've submitted the ADSAAS application and transcripts (all of that stuff takes FOREVER to process.)
So basically, give us more information about the specifics of your situation and we'll try to help.
(On the other hand, if you're applying for 2010, you have nothing to worry about.)