I also turned down an invitation in 2008, reapplied in 2013 and was invited. Then, about four weeks before staging I was pulled from the group for medical clearance (wasn't able to get a test done that PC wanted in time - even though doctors didn't think it was completely necessary). Since it was at the beginning of the transition to the new system, I was told to reapply rather than be put on medical hold, and now I am once again under consideration.
No major issues with any of it, other than I had to write a statement to explain why I withdrew in 2008.
Also, keep in mind there are no nominations anymore; when you select a country during the application you are (hopefully) put under consideration for it and will interview with that specific country placement officer, rather than a recruiter who then nominates you. I know you read a lot about nominations on the boards, but that is leftover from the old system.