When you launch Minecraft you will be promted to update. Make sure you click Not Now. If you update, you will not be able to play on the server until the server is updated and this will likely be a couple days from now, I’ll post here when I know more. Before I can even begin testing/configuring server for 1.3 (step 1), I need a stable version of Bukkit. Check Bukkit 1.3 progress here.
Bukkit Twitter updates here.