I know this can be done, and has to be done. For server admins who want to have any type of real control over an environment, an economy and their Minecraft World at large... as long as there are client side components that do not jive with your plans as a server admin, you will be at the mercy of cheaters, trolls and hackers. A Pure Client Check would make a request upon client connection for an md5 checksum of the client code. This might have to be a core feature of bukkit rather than a plugin. Think of it as PunkBuster for Minecraft. The goal would be to eliminate the use of modified clients to prevent things like ore detection. This would be server configurable for admins who just don't care. This could also include an option to force usage of the default texture pack (to eliminate invisible block packs) or a selection of approved texture packs. I hope someone picks this idea up... I would make an effort myself but I barely find the time to play let alone code.