A GF of mine did for her DD's first bday and I thought it was sort of odd. THEN, when my DD's 2nd bday came around, everyone started calling me for suggestions and asking me what does DD REALLY like? So I turned around and did the same thing.

I didn't include it in the invites, I thought that was too much. But in the invite I did put our website that DH created just for DD's bday party info. On the site, there was info on the party, directions, photos of DD, and then I put a little note saying:: Thank you to everyone asking what special gifts they should get for DD, to make things easier, I've created a "Wishlist" at so and so site. You don't have to buy from the list, but you can use it for gift ideas. Thanks again for wanting to make DD's bday extra special!" Or something to that effect.
I went to Oompa.com and I made a wishlist for everything that I think DD would enjoy. Like puzzles, puppets, books, etc. Since there were SO many toy recalls recently I was very picky on which toys she could play with.
We only invited family & very close friends only so I wasn't worried about anyone taking offense b/c they'd tell me straight out. Had I invited play date pals or class mates, I wouldn't have given them the wishlist link. Everyone used the wishlist and afterwards, so many people pulled me aside and said thank you for creating that wishlist, it took the guess work out for me!
HTH, good luck w/ the planning!