Some of my favorites:
"I was worth the wait"
"Future Scrabble Champion"
"If you can read this, you are close enough to change my diaper"
"born to be worn"
"my fingers may be small, but I've already got daddy wrapped around them"
"I traveled through the birth canal and all I got was this lousy t-shirt!"
"Please don't give my mom any dumb advice -she's doing just fine!"
"Mommy's coming! Quick! Look innocent!"
"Breastfed baby - stick around for the show"
"Good Looks Run In The Family"
"I do my own stunts"
"My IQ is higher than the President's"
"Mommy Drinks Because I Cry" (I sent this one to my SIL's sister who's baby cried non stop for 3 months - she loved it).
There are some pretty crazy ones on tshirthell.com but I will warn you - there are some really offensive shirts on that site so if you don't have a thick skin - definately stay away.
My favorite is the grown-up t-shirt:
"I make milk. What's your superpower?"