And here it is. It's an standalone perl script and is invoked as follows to list all installed packages and the repository they came from:
You can also give it the name of a repository, in which case it will only list packages from that repository:
If it doesn't recognise the repository name (it should match those in App Manager) then it'll print out a list of the defined repositories.

Packages are listed using the package name and, where available, the display name. If the version installed cannot be found, then the version number found will be printed as well. If the package cannot be found in any repository then "Unknown" will be printed.

Let me know if you have any problems running this - it seems to work fine on my N900 but there may be unexpected issues on different firmware variants (I hope not - it should be pulling out the correct variant URL to use).

EDIT:
- Added documentation to zip file
- Added check for disabled repositories
- Added prioritisation of maemo.org repositories, so only the highest-priority match will be reported
- Moved script to my webspace: http://www.robinhill.me.uk/maemo/package_sources.html