Good news: you shouldn't have to wait through the entire month. If you've pre-ordered GSM 2003, you should receive your copy no later than the second week of January (probably earlier, but usual disclaimers about anything going through the post office apply, of course). It should appear in stores and newstands on or about the 15th.
If you enjoyed the 2002 edition, you should find the 2003 GSM even more mouth watering. The emphasis is on dioarmas this year, so not only will there be a ton of tremendous models but you get to see the ingenious ways modelers come up with for creating realistic settings. Okay, I'm pretty excited about this: it was so much fun putting the magazine together!