I have a forced hot water (baseboard) heating system in my home and added central air conditioning about 10 years ago. The cooling system's air handler is in my attic, and duct work was added for the 1st and second floors for cooling. There are 3 returns on the 2nd floor, and about 10 supply vents throughout both floors.
My question/issue is that two of the returns and 3 of the supplies have pretty bad (cold) drafts throughout the winter. I measure the temperature at around 50 degrees F on them. All the others are at/near room temperature.
I'm considering blocking them for the winter months by purchasing covers, or likely a DIY project like http://www.instructables.com/id/Winter-HVAC-Hack.
My main question is that should there be any drafts at all. Are the drafts a sign of duct leaks that should be found and repaired by the HVAC company?
The air in your duct work is cold because it is running through the attic which is not heated. There will be some air flow through the ducts because they are not a "sealed container" they are designed to move the air in your home.
For the ceiling registers you will want to make sure the magnets are strong enough to hold material tightly.
I had the same problem with cold drafts coming thru the ac vents that are not used for heating in the winter (I too have baseboard hot water heating and only use the ac and ducting in the summer).
I have 3 returns and a lot of supply vents and covering each one individually would be time consuming and not practical.
Instead I pulled out the filter from my air handler (which like yours is also in the attic) and covered it with plastic sheeting. This blocked the air circulation between the returns and supply vents and eliminated the drafts.
I have not heard of others using this method but it worked very well in my situation.