Problem solved. Now we do a PPD update during configuration of the "cups" package, to update the PPDs for all printer driver packages where we missed the triggers due to the fact that we also update "cups" at the same time.
See also http://www.seanius.net/blog/2009/09/dpkg-triggers-howto/, section "How triggers work in a nutshell":
Note that if a consumer is going to be normally configured (i.e. it is also being updated), then no triggering may occur and thus the standard control flow of the maintainer scripts should still take care to handle this.
The consumer in our case is the "cups" package and the producers are the printer driver packages.
The fix is in the upcoming CUPS 1.5.2-6 package which is currently in Debian's BZR repository and will soon get uploaded to Precise.