Congrats on the wedding!
WTSGB is a pretty song. However, like Bonochick said, bear in mind that it's not the happiest song if you listen to the lyrics & the meaning. That's the problem with almost all of U2's love songs---they're all bittersweet. We had thought about using All I Want Is You, until we really thought about what he's saying in it (you want diamonds and all these crazy things, but all i want is you). What we ended up doing was dancing to another song ("Songbird," sung by Eva Cassidy), and having the DJ play the second chorus from Beautiful Day when he introduced us to the reception. That actually worked VERY well!