When you land in JFK (or any U.S. airport) from overseas don't you basically "enter" the country, meaning passport control, customs, and then airside.

So if you are continuing you HAVE to go through security again ANYWAYS?

This being the case, even if they do NOT tag your luggage through to BOS it isn't that much hassle/trouble to go to the normal ticketing area and check your luggage in (assuming the lines aren't bad).

How long would your connection be in JFK?

Also, if BOTH flights are on Delta it should be possible for you to call Delta and have them link the PNRs, which would help you get your luggage tagged through to BOS in London.