Maybe you'll know the gender by then!!! Well if you find out the gender anyway. I think sending them a home video of the sonogram would be so so so sweet!
I agree....do some kind of gesture that way. You could send them bibs in the mail that say "I love my Grandma/Grandpa". or something like that.
We told our family for valentines day and made up cards. You could always think up something cute for thanksgiving since that will come up soon and will be before Christmas.
Maybe sending flowers, or making a home made video?
send them the ultrasound picture inside an very early christmas card.. and say something like an advanced christmas gift... :p just an idea...