Congratulations! We usually are on the island in Sept. and the weather is usually perfect, but as Jeff says, you have to be ready for possible rain.
My daughter was married on the island Nov. 24, 2007. (see trip reports for some entries about that day). We chose not to use a wedding planner since it was a small wedding--less than 30--and we had it at the Hilton Head Plantation House. We worked with Artis at Tapas and the buffet was delilcious; signe's bakery--she was able to replicate what my daughter wanted perfectly; A Floral Affair--loved working with the owner. Weddings are expensive but we felt everything was priced fairly, and they gave us great ideas on decorations, etc. that we were able to make from supplies at Michaels. Good luck!