Worldguard received a minor update today which allows me to prevent mobs from spawning on selected lots. So I disabled mob spawning completely in the spawn areas of all 3 worlds. This does not prevent mobs from other lots to walk over into those areas, which might be an issue in the nether. The other areas should be fine though due to size or walls.
As you might now, Notch has announced minecraft 1.5 for next week. On the other hand I announce holiday 2011.3 next week. This means that I will be off for the second time this year after XMas and Chinese New Year. I will be going to some obscure island in Indonesia for diving (yay!) with some limited internet access (nay!). Which means: I might have trouble updating the server in time if the plugins are quick updating or even to setup a parallel server for those who upgrade their client.
So rather wait with updating your client until you see an announcement here that a parallel server is running OR that your old client tells you it’s outdated when you try to connect.