Under the What's new for geodatabases in ArcGIS 10 Esri talks about an optional parameter that allows you to create a 9.3 GDB [GeoDataBase] in version 10. I can not find any other info. Any help on how to get to this "optional parameter" working would be great.
Swingley's answer should be the accepted answer, but I'll offer this up as an alternative since I know this to be common: Make a copy of an existing empty 9.3 geodatabase.
At each ArcGIS release, I create an empty file geodatabase and empty personal geodatabase and store them on a network drive and on the USB. Whenever I need new one, I just copy a geodatabase from my stash.
As I was writing this answer, I remembered James Fee's blog post about the same topic back in 2006. It looks like he's actually added links for 9.3 and 10.0 geodatabases to the blog post:
It's an optional parameter when using the Create File GDB geoprocessing tool.
The ArcGIS version for the geodatabase to be created.