I believe SRV records in your DNS zone file would do the trick ...
http://en.wikipedia.org/wiki/SRV_record
 
 
In your DNS zone file use something like :
 
www1     14400   IN      A       1.1.1.1
www2     14400   IN      A       1.1.1.2
www3     14400   IN      A       1.1.1.3
www4     14400   IN      A       1.1.1.4
www5     14400   IN      A       1.1.1.5

_http._tcp.yourdomain.com. IN      SRV 0    5      80   www1.yourdomain.com.
_http._tcp.yourdomain.com. IN      SRV 0    5      80   www2.yourdomain.com.
_http._tcp.yourdomain.com. IN      SRV 0    5      80   www3.yourdomain.com.
_http._tcp.yourdomain.com. IN      SRV 0    5      80   www4.yourdomain.com.
_http._tcp.yourdomain.com. IN      SRV 0    5      80   www5.yourdomain.com.

Since all 5 hosts have the same weight and priority, you should get a 20% spread across the board.