while the legal situation is not entirely clear here, as I read the law you will still have fulfilled the qualification for opting out if your total salary for this year including the one month when you are below the threshold will exceed the total threshold of 48.150.- EUR gross salary for 2008.
hope that helps. if your employer (who has to judge if you are allowed to switch to private
health insurance or not) is uncertain about this issue and needs reassurance/help, just get back to me directly (see the link down below at the disclosure line) and I'll be happy to explain this to them. Some German HRM folks are still very uncertain about the extend of the change in law and the regulation therein.
Before you actually opt out, however, you should get qualified and independent advice on this issue to make sure that opting out is really your best decision.
Cheerio