Recently switch all of the WordPress.com load balancers to be nginx. They push a little over a gigabit of traffic right now and about half a billion requests per day, no sweat. We use Spread + Wackamole for failover, there's more info on Barry's blog:

http://barry.wordpress.com/2008/04/28/load-balancer-update/

I wouldn't recommend DNS round robin for load balancing. (We did it for a while, many problems and flaws in the approach.)