Hmm, according to Wikipedia there is an alternative date of 23rd December:

http://en.wikipedia.org/wiki/2012

So, I think there are two important points here:

1) No-one knows what time of day this event is supposed to happen (if you know, don't forget to offset for the time difference between South America and Holland) (Oh, and if you're sure, please update Wikipedia. Thx).
2) If nothing happens on the 21st then hold tight for the 23rd, just in case.

I'll be in New Zealand. Probably the first country to see the dawn of that date. Will try to send a message to this forum before the link to the internet melts down.

Of course, on the positive side, it means we won't have to fix the Unix 2038 problem:

http://en.wikipedia.org/wiki/Year_2038_problem

A