I have to admit, I'm a bit mixed on requests to give to a charity in lieu of gifts for a wedding or baby. On the one hand, I think it's a nice way of saying "our marriage is what matters to us - not getting material goods we really don't need". On the other hand, it kind of plays into the notion that a gift is expected, compounded with an assumption that how you give that gift can be dictated.
Sometimes, people want to give a gift, and make it a keepsake gift, because you are special to that person and they want you to have a momento to signify that. I have a couple of dear friends that when they got married, I was very careful in selecting a gift that would be a keepsake that would stand out, and not just a setting of china or whatever, so that 25 years from now, they could look at that item and remember where it came from. I also have some things I've been given on special occasions that I still look at and remember who gave them to me and why, and it makes it special (the earrings I have in today as a matter of fact, came from my mom's best friend for my high school graduation) In those cases, asking for charitable donations in lieu of gifts gets tricky. If I give you a gift instead because you're special to me and I want you to have a keepsake, am I being heartless in not supporting the charity?