Basic there is something in development currently called DynCraft its aim is bring an API in so that SP mods can work on SMP seamlessly with out any mucking around with the Client files and installing mods and so on and so forth. It sounds very interesting and he appears to be making a bukkit version of this as well at some point. Read about it Here! What I can gather from this info is that it would allow servers to host SP mods and the clients would have no trouble grabbing the mods and installing it. Personally I think MC should have had this functionality long ago but notch is only working on an API now and I doupt its ever going to grab mods on its own so you will still need someone to create a launcher that does this when joining servers. But this seems interesting none the less.