I also had a 'false positive' on a PET scan about 4 months after I completed my first round of treatment (IMRT X 35 and chemo). The PET indicated FDG uptake in the tongue where my tumor had been. A subsequent biopsy revealed there was no recurrent cancer there, it was just an area that was still healing. As mentioned by others, areas that are actively trying to heal can also light up a PET scan just like cancer does.
-Steve

Age 41 - Stage 2 SCC tongue Dx 2/06. Cisplatin x3, IMRT x35. Mets to neck node discovered 7/07. RND 40 nodes removed, margins not clear. Cisplatin, Taxotere, 5-FU Fall 07, then IMXT/Erbitux for 7 wks. Inoperable mets to both lungs and pleura Dx Oct'08. 4 cycles Carboplatin, Erbitux, 5-FU so far.