I found
this great chart which lists recent national trends:
Year Number Accepted Number Applied Acceptance Rate
2003 17,539 34,786 50%
2002 17,592 33.625 52%
2001 17,455 34,862 50%
2000 17,546 37,136 47%
1999 17,445 38,625 45%
1998 17,379 41,033 42%
1997 17,313 43,020 40%
1996 17,385 46,968 37%
1995 17,357 46,591 37%
1994 17,317 45,365 38%
1993 17,362 42,808 41%
The few schools I've seen that publish data on their undergrad students acceptance rates seems to indicate that first time applicants have around a 10-15% higher acceptance rate than second time applicants.