Fix boot splash to greeter transition
Plymouth stops before GDM is shown, displaying a black console with init messages. This is only visible for a short time on a quick computer, but on older hardware (and probably while booting on CD), it could stay several seconds.
The first lines displayed are:
bc. INIT: entering runlevel 2
Using makefile-style concurrent boot in runlevel 2
This issue looks like a bug mentioned at the end of Debian bug #575431, now unreproducible by the OP.
Tried updating to keyboard-configuration and console-setup from sid, with no success.
wait for the migration to Wheezy + systemd, that may magically fix such things.