mrobinso - your car must be different than mine.
Mine clears up fine and fast.
However, if the condensation is left in there, and can't get out, the heat from your bulbs will turn the condensation into a film on the inside of the headlights.
Its not the engine heat that clears it up. If you read the TSB of Jan 13/05, the suggested fix is to remove the caps and leave the headlights on for a minimum of 45 minutes, and then install new vents.