jeb_ has recently made the world aware that Minecraft 1.5 is now available, however we’re not ready just quite yet.
Please do NOT upgrade yet! If you do so, you may be unable to access your CraftBukkit powered server, as the Bukkit team has not updated CraftBukkit/Bukkit yet.
If you do upgrade, keep in mind that you’ll need to upgrade your server in order to play on it. Since CraftBukkit is not yet available, you’d need to change your server to vanilla Minecraft.
Plugins will not work, if you switch to vanilla, and anyone will be able to build on your server.
In short, unless your server is running on vanilla Minecraft, we highly recommend against upgrading until Bukkit releases an update for CraftBukkit.