http://weblog.sitemeter.com/2006/09/02/statistics-currently-delayed/
My guess is that it's related to yesterday's problems and just figure that it'll update in a few more hours.
I've found that sometimes SiteMeter has technical problems, and hits aren't counted, but this is rare. If you're using their free service, you get what you pay for. Couple of things to check:
1) Make sure you've properly placed SiteMeter's code on your pages: http://www.sitemeter.com/?a=manager&area=htmlcode
2) Read the FAQ. There's some good info on trouble-shooting: http://www.sitemeter.com/?a=help
3) If all else fails, try contacting the administrators: http://www.sitemeter.com/?a=contact