Testing Ubuntu 10.04, I'm happy to inform that you can easily process your old Kodak DCS files using Raw Therapee (http://www.rawtherapee.com). Had no problems processing TIFF files from a DCS420 and a NC2000e, but I was unable to open a DCS720x DCR file. Image quality was just as good as other software I've tested, including Kodak Photo Desk (or even better).
NC2000e (default settings):
DCS420 (minimal tweaking of exposure and contrast):
Jarle