I have also encountered this problem several times, and although this list seems to recommend running clvmd -R it has in fact never helped the situation (I'm running centos 5.2). The only way I can solve this problem is by rebooting all nodes in the cluster and then extending the lv.
We have seen this too, but do not go the route of rebooting. The nice thing about clvmd is that it's not required for the cluster to continue running once up and established. It's purpose is to communicate LVM metadata changes to all nodes in the cluster. So you can simply kill -9 the clvmd process on all nodes then run service clvmd start. This will get clvmd back up and allow pv/vg/lv commmands to complete correctly.