Many tales were of dragon's have been in some extremely cold climates. Given that reptiles are ectothermic (or as some prefer "cold-blooded") I find it rather impossible that dragons would be a variety of reptile.
I can imagine them regulating their own body temperature however. This would mean they'd have a higher metabolism.. so you could go on from there to ponder what their main food source would be if they were to eat so much.
You gotta love how all these questions chain off each other..

Thoughts?