Since last weekend backports.org has two more services supporting tracking of what's going on. I will list them in chronological order:
Tracking of Security issues in backports packages: This was one of the many topics that was discussed at the Security Team meeting in Essen at the end of last month and Florian Weimer implemented a first version of tracking security issues within backports.org. It currently compares the backported version against the one fixed for unstable, so at the moment it still has some false-positives (e.g. libspf2 because the fix was taken from lenny-security), but this is still a big step forward and helpful to track outstanding issues here, too.
Diffs between etch-backports and lenny: Something similar was available at some other places before but strangely got discontinued. In here you can see which packages in backports are older (meaning only debian revision difference), outdated (newer upstream version in lenny) newer (backported from unstable?), have a wrong version schema or are not available in lenny (propably even removed from unstable). It will hopefully help people to get their backports in sync. At least it's an indicator of how well the packages are tracked.
Hope you consider them already as useful as myself, even though there is obvious space for improvements. But they are both quite helpful already in their current state so that shouldn't hinder you from using them. :)