I recently set up my Brother P-touch QL-550 label printer to work with CUPS on my laptop, which runs Ubuntu 9.10 (karmic). This required installing ptouch-driver, with some patches, and then hand-editing the PPD file that gets created.
I made a very crude .deb package to install the ptouch-driver. I have no idea how to package CUPS stuff properly, so I didn't even try. I don't intend to maintain this in Debian or Ubuntu, I just needed it for myself, so I didn't go to much effor, I'm afraid. Perhaps it's useful anyway. I filed a "Request for Package" bug in Debian (555892) and talked with Till Kamppeter from Ubuntu about this, and he has taken a look at this (see comments in the bug).
With my package, the generated PPD has some margins that make it almost impossible to fit anything on the label. The margins are only a few millimeters, but on labels that matters a lot. I fixed this by hand-editing the PPD, Till seems to have fixed in a better way.
I hope this is useful to someone. The printer itself is pretty nice, too bad Brother doesn't provide free drivers.