Launchpad's bug tracker is unique in that it allows you to track the status of the same bug as it affects multiple different communities. This gives you a single picture overview of the work that is ongoing in all of those communities to engineer a fix to the problem.
Here's an example:
attachment:multiprojectbugs.png
Looking at this bug report, you can see that it is currently affecting at least 3 communities:
- Java upstream
- Debian
- Ubuntu
In the case of Ubuntu, the bug has been recorded in two places - the sun-java5 and sun-java6 packages.