According to the papers today, 9th January is the date that most people pack in their New Year's Resolutions 
I'm on the wagon until after Brighton Marathon, I cut out alcohol when I was training for half marathons and found it really helped my training. Between Christmas and New Year I managed to do several long runs on days following evenings when I had basically marinated myself in booze, but I had the luxury of being on hols from work so I could sleep off a hangover instead of trying to run through it.
For me, its not so much the booze as the wanting to laze eating comfort food the next day that ruins my training plans ... I can't afford to lose days of my training to a hangover and I find it easier to say "I won't have any" as "I'll have one ... ok make that two ... who wants tequila shots?" 