Back when supercoley posted here more frequently, he told me that it was about whether the ballast is programmed or quick start. Programmed last far longer, but T5NO and CF are almost universally quick start, where as T5HO are programmed. He used programmed start T8's for a lot of his lighting. The reading I've done on the side from a lot of commercial sites seem to agree with his line of thought, though I found it almost an impossibility to get good information from a source that wasn't toeing a company line.
-Philosophos