They always seem to see the errors of their ways don't they, no way they'd ever just pick a new date and ignore their old one </sarcasm>.
Well considering the entire theory is from the Mayan books, I don't think much will change. Personally they will most likely just claim it's "Gods will" that we all lived and beat their religious scripture of choice with an added vigor come January 1st 2013. HOPEFULLY people will start to see it's all bullshit though or at least tell the Extremists to pipe down and will no longer fall for the whole "doomsday" scam AGAIN.
Also I'd like to point out that the Mayans believed the world ended 4 times before and that this is the 5th Earth. They had the same type of calender we have today, the 5th of March comes and goes every year no matter what. It seems to me that in their own way the Maya had a sense that History repeats itself, possibly why they choose an exact date for the world to end.
2012 could just be the end as we know it - meaning massive change in a the course of 365 days, 366 days should 2012 be a leap year. Some theorize it will be the dawning of a mass "awakening" within all of mankind. Sort of follows the old saying "All men are brothers" which would be nice if one day everyone woke up and was accepting of one another. Now I'm not saying I believe the great sky serpant is going to save us from ourselves while we all sit back and be lazy good for nothings, buuuttt... wouldn't that be awesome? LOL sarcasm.
We won't know the answer until that day comes however... but just in case I'm going to play "Last night On Earth" by Powerman5000 and "Who Wants To Live Forever" by Queen *a live version from Budapest in 1986* on new years. If the world does end I'll be that big German guy in Southern California playing this loud as hell and singing along with it to freak out the Religious Right: http://www.youtube.com/watch?v=zeo0_3gN190