I currently have a huge problem. Two days ago my site running on one server was too much, so I purchased two more and had them clustered (rsync and load balanced).
I
You want to be careful about doing this. One thing to be careful about is to not host session info with other non-session data. It isn't the biggest deal to clear your cache when it only contains your own site's data but you do not want to wipe out people's sessions along with it.
As long as your are using the same key with memcache you should hit the same server every time. So that issue should go away.