I found a really cool site (geared for DJs) that lists a variety of top requested music. The link is: http://www.discjockeys.com/top_200.php
The lists include:
-Top Wedding Songs
-Top 200 Requested Songs for the last Year
-Top 100 Songs for 2008
-Top 50 songs for various decades
For those of us doing our own music (ipod ceremonies for the win!!), this gives you a great starting point and can help fill in some of those blanks!