I don't know about you but I found the out of the box fly-out navigation depth (1 sub-level) a bit basic.
Here's how to customise the depth setting:
The depth setting is in the SharePoint:AspMenu tag and it's name is MaximumDynamicDisplayLevels.
You can open the default.master page in SP Designer or alternatively open it on the file system at [drive]:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\GLOBAL
In the AspMenu tag (ID="TopNavigationMenu") you'll find the property MaximumDynamicDisplayLevels set that to your required value and that's it, done!
Don't go too daft though or there's the potential for some significantly bad UI!!!