#2. "RE: D600 RAW Files and LR 3.6" In response to Reply # 0
The upgrade is the most convenient method. The free (sort-of) method is to download (free) Adobe's DNG converter and convert your D600 files to DNG. LR and PS going back several versions will read DNGs. It is not free in the sense that you pay in time, some inconvenience and, if you keep your NEFs, more occupied disk space.
All that said, LR4 is several steps ahead of LR3 and you will probably appreciate the upgrade once you wrap your mind around the new regime in the basic module.