Minecraft version 1.4 could be out soon. When it is released it could take anywhere from hours to a week+ for the server software to be ugpradable. The bukkit developers have to patch their software and mod developers sometimes also need to update before i can upgrade to 1.4 on our server.
I recommend holding off on upgrading your client to 1.4 until you get the 'client outdated' error when logging in, that is the signal that the server is upgraded
another alternative is to copy the .minecraft/bin directory to bin1.3 and upgrade, that way you can restore 1.3 again if you want to log back in on 1.3 servers
keep in mind most of the 1.4 servers that are immediately available will be vanilla servers. that means no mods, no enhancements, no protections, and no optimizations. could be fun, might also be agonizing
