Lisa - You just made me realize that my SIL's kids do the same thing!!! Richie has outrageously good manners - after he opens a gift he puts it down, and goes over to the gift-giver and says thank you and hugs them. Andie of course, is only 1, but as soon as he is able to will learn the same habit. It is of course, only because we practice what we preach - Randie and I do the same thing.
The day after any of us receive a gift - a thank you goes out in the mail. Richie has just learned to write "thank you" - and he draws a picture to go with it.
Every gift deserves a thank you - but I do continue to give them. I guess I just stifle my irritance.