A new MOSS SDK is available, and it comes with a free tool to create BDC definitions (is this the end of MetaMan? I wonder...).
The new tool is called "Business Data Catalog Definition Editor"
MSDN already have articles on how to use the tool with web services and other systems.
Installing requires you to install SQL server 2005, so don't install on your production server (duh!), and be prepared for some drastic changes to your system when you install on your dev box.
The good news is that it can be installed on windows XP, proving that Microsoft are listening to us developers, and to our gripes about developing on a server...
I tried installing by running the MSI directly, but that didn't work (didn't install the SQL express, and the installation failed when it couldn't find SQL express).