Just got a clarification from PADI Americas. While the document mentioned has the wording as already given in this thread the official position is the release from the physician must be within a year. This makes things crystal clear.
"27 Oct 11
With respect to your question, a medical & doctors approval are good for a period of one year provided as you mention no changes to their medical history. This standard is actually found in the Instructor Manual which you can download for free from the PADI Pro Site. Regarding your second question, no the medical form and duration of its validity are the same for all PADI courses for the Americas Office and region. There are differences in other countries which fall under different PADI Offices.
I hope this answers your question.
Regards,
Steve Mortell
Consultant, Training
PADI Americas
For completeness my question to PADI read "A question came up as to how long a medical release is valid for. Reviewing the RTSC form there is no time frame specified on it. Clearly if there are any material changes in health a new form is needed, but if there are NO changes, but the diver had to answer YES to one of the screening questions, how long is the medical clearance good for. Is it different for OW or Con-Ed?"