It does vary, January is not predictable.
we spent Jan 13 - 18th at the grand floridian last year (2001) and we had great weather, sunny and low 70's every day (however the week before that they had frost!)
this year we were at BWV for marathon weekend January 4 - 8th. it was freezing cold! it went down to the low 30's at night, and it was only up to about 60 during the day.
florida usually has about 2 weeks of very cold weather around end december/January. I hope you have good luck with the weather!
I'll be at BCV this january for the marathon, Jan 10 - 14th, I hope it isnt like it was last year for the marathon, I froze my butt off waiting for it to start!
have a wondeful time, and pack for all weather, 30's and 80's,
you'll have a great time no matter what though, you'll be in WDW!
and the GF is fantastic, you'll love it.
Laura