This is really one of last functionalities we're missing in redmine.
We need to have time reports across all projects.
I know there was already some discussion around this, but I cannot find related tickets now.
Also there was idea to move this functionality to Time Sheet plugin, but I really think that it will be better to incorporate this directly into Redmine for several reasons:

• Much faster development of Redmine than development of plugin
• Current reports are just great, I don't know object model exactly, but I thik it's just a step from this functionality
• All future improvements will be applied to global reporst if it will be park of system and not stand-alone plugin
• Plugin do almost the same what is already in Redmine, so it duplicates functionality a bit

Anyway, there is suggestion how it can work:

• Project specific report is: http://redmine.example.com/projects/projectname/timelog/report
• Global report URL can be: http://redmine.example.com/timelog/report

This will help us to track what all our developers doing in perspective of day and week, so it will dramatically improve project management flexibility.

PS: We already developed our hack for this, but because we're PHP developers, we have some PHP hack which looks up infromation from redmine's DB.
If anyone is interested, I can post some screenshots.

PPS: Because time-reporting is critical functionality for our project management in our company, I would like to prepare some more imrpovments on how reports can work.