According to BBC reports, "About 3,500 hectares (8,500 acres) of pine forest and at least 15 buildings have so far been destroyed in the blaze".
Halkidiki is a very popular tourist resort and May 2007 will mark the start of next year's summer season. With that in mind, I think it is very fair to say that any necessary repairs or building work will have taken place by then. I don't think Greece could afford for tourists not to visit.
But bear in mind that the damage is on a relatively small scale. Look at Phuket in Thailand for example, following the Boxing Day 2004 tsunami. The damage done there was devastating, yet the island seemed to get turned around very quickly and was open for business again not long afterwards.
I think you will be completely safe booking a trip for May 2007.