Growl 1.3+: Location of (rollup) Log for use with GeekTool
Growl uses a CoreData database located in ~/Library/Application Support/Growl/

The database is a file called notifications.history

Edit: there does not as yet appear to be any way to communicate with the Growl Control Suite say via AppleScript, to read the database.  The functions there are limited. See the Growl AppleScript documentation here

You would have to communicate with the database directly, a database model is included within each build of Growl.