If you're mining through slush's stratum proxy (e.g. with ASICMiner blades or cubes), there's a known problem with p2pool (and this gets passed through Multipool) - it won't work with that proxy.  Some have suggested that it's best to run with the bfgminer proxy, but I'm not sure if that gets around the p2pool problem since I can't run it more than 6-8 minutes before it declares the devices SICK and restarts them, killing my hashrate.  This latter is a known problem with certain devices, wireless, and QoS - it's a tough nut to crack, and I haven't had time to troubleshoot.