As Necrodoom said, it won't be a good idea. For example, let's say you have a Bukkit server with a world generated with 1.8. The 1.7 and lower clients will have no clue what the new Guardian mobs are or the new blocks/textures. The client and server will not be able to communicate correctly. And the reverse is true too, if you have a Bukkit 1.2.5 world/server, a 1.7 client will not work very well either because there are a lot of gameplay changes as well as underlying network communication that has changed. LHammonds