Each Project is created as a separate USER (if using Oracle. However, if you were willing to use a 3rd party reporting tool, such as Crystal and set up a data dictionary that interrogated the multiple USER projects creating a single data source, then a report could be achieved.
The issue of cross-project reporting comes up frequently, since our project structure is time based. Initially, we were going to build a VB application, but the effort was pulled back in light of a better solution. That solution was to copy all test assets up to a central project repository.
The problem with this method is that TD has never been tested beyond the 30K defect count. We began to hit limits and decided to move away from this structure.
In my recent development efforts, I created an advanced SQL backend stored procedure that enables cross project report. The front end is ASP, allowing for any project manager in our global environment to execute a report without requiring special installation scrips or DLLs. This solution is thus far the most powerful and efficient custom tool I've ever heard of for cross project reporting.
If you would like to discuss this in further detail, please contact me at email@example.com.