Thank you, Anton, but linux-restricted-modules is no longer being used in karmic.
I have solved the problem by removing linux-backports-modules-2.6.31-1-generic which contained the bogus lib80211.ko. I have no idea why I installed that package in the first place. Now wl.ko loads correctly (and automatically) and its dependencies are correct:
Please check whether the failure I described is the expected behaviour and whether a “Conflicts:” should be added to avert this issue. Otherwise, just close this bug. :-)