I know this is an old post but thought I'd reply anyway:
Yes for simple configs DNS/GSLB in front, or something like Amazon ELB with PROXY PROTOCOL support for IP transparency.
But for anything complex like persistence with stick tables etc. then you need a pair of HAProxy nodes with a floating IP address:
You can use Keepalive which has already been mentioned (which lots of people like) and its very mature.
However we currently use Heartbeat from linux-ha.org - which is ancient and therefore pretty solid.
But anoyingly it's designed more for two servers with one sharred storage and STONITH (Shoot The Other Node in the Head).
Rather than load balancers where going active/active is not a problem.
AndyZaks PulseHA project (pulseha.com) is lightweight, fast and specifically design for load balancers.
He started the project because Heartbeat and Keepalived can be a right pain when they split brain (loose the heartbeat).
We're currently helping him add the finishing touches to it for our next gen version of Loadbalancer.org appliance.