Congrats!
A friend of mine waited until she got a decent ultrasound profile picture and put it on her computer (scanned, I guess). Then she made a TShirt with it and put in "Hi Grandma and Grandpa!" on it.
I thought it was pretty cute.
Her parents lived in another state though and they were able to keep it a secret for a couple months until a face to face visit. Might not work for a lot of folks.