I had already tried the proposed solution of letting a CD in the drive to no use, it still opens randomly.
I just tried the eject command: eject -i on /dev/sr1
with sr1 being the offending CD-R in my system. It has no effect on the eject button of sr1, but instead it blocks sr0, the DVD!
And no, if I do eject -i on /dev/sr0 it does not block anything.
But anyway, this are unacceptable tricks for a serious OS like linux should be. This is happening just after an upgrade on perfectly running hardware since Ubuntu 7.10.
Any more ideas? TIA