Vic is correct. There is a Volvo Technical service bulletin describing the revised vent cap "fix". You can view it at http://www.nhtsa.dot.gov/TSBScans/sb616735.pdf

The good news is that the caps may be replaced under warranty (but only if your VIN matches the service bulletin). The bad news is that (1) your dealer will deny any knowlwdge of this service bulletin, so bring a copy; and (2) even if you buy the caps and replace them yourself, they don't make much of a difference. I've replaced the covers and in winter, water still condenses in the housings immediately after the car is moved from a warm garage to outside. It helps to turn on the headlights for a minute or 2 before you start your car.

An unfortunate consequence of the condensation is that it leaves behind a coating of mineral salts (that precipitate out of the miosture in the air) on your headlight bulbs, reducing both light output and service life. I have to change my low beam bulbs twice a year (I buy them by the half-dozen; not a big expense, just an annoyance).