From what I have heard, the more attached they are to you NOW, the easier it will be for them to be independent and self-sufficient when they get older.
At the risk of sounding "crunchy" , I think that it is perfectly healthy and good for them to be attached to Mom or Dad. Tyler was very much a momma's boy when he was a baby and even though he still prefers me he is also very independent and has no problems playing by himself. I believe that his attachment to me taught him that I will be here for him no matter what so now he feels comfortable branching out because he knows that that is still true. Does that make any sense??

I'm not saying that a child that doesn't form a strong attachment doesn't learn that, but I certainly think that that attachment is beneficial