If you use this, your customers' data is now in cleartext between the ELB and your instance. Now, that may be fine, you're already trusting EC2 if you keep cleartext data on disk. But it's worth keeping in mind.
I haven't tried this myself and the docs dont say if its (im)possible, but it seems like that'd be a good feature.
Much better: Forward via HTTPS to EC2 instances which are using short-lived certificates. When you boot a new instance to add to your load balancing cluster, have it generate an SSL key which ELB is told to trust. If a node is compromised, you just tell ELB to not trust that certificate any more.
But that doesn't seem to be possible, and it still doesn't make up for the fact that putting SSL stacks and $BIGNUM SSL keys together is practically begging to be attacked.