Thank you for the input! Tif itself seems to be cross-platform compatible, I have no problem modifying the files in PS CS2 on the PC. I just don't like the workflow (only use PS for specific modifications, it's not part of my normal workflow, too heavy). It's just Capture NX2 telling me it can't do a thing with this file...
Since I work on a Mac, I think I will switch to Aperture entirely. I have tried the program, and I like the workflow. Nikon software has its advantages, but its limitations too.
Thanks for the feedback, Holger