In reply to Dipsterdesigns:
I'm assuming that you're talking about an old injury, rather than trying to climb with a recent, still healing injury??
I ruptured my ACL about 4 years ago, had it repaired and have had no real problems with it. I had surgery on it to repair the ruptrure and a fair bit of physio / waiting before I got back climbing though. I ruptured in it January, had the op in March and I think was back climbing July/August or so, and hillwalking in May.
Only probs I've found is that sustained days cramponing / step kicking starts to hurt, as does carrying heavy loads for days on end. That said, I'm sure a lot of folk with no ACL history find the same!
If you mean can you keep on climbing while recovering - I'd err against it. I remember how unstable my knee was, and how easy it was to do more damage. Advice I got from the sports physio was that while in recovery climbing wasn't recommended due to the odd angles etc you load the knee at.
Once healed and the joint was back up to strength no probs at all though!
Cheers,
Stuart