Check to make sure the seal at the bulb is good and tight. Occasionaly, espically if you've replaced bulbs, the bulb won't seat properly or fully and will let moisture in. It's something that may fog up today, and not tomorrow, depending on weather, sun, etc. More then likely it's just a bad seal between the headlamp housing and the lense. A kind of hot glue is used to hold the two together, and if there is a dry spot, after time water can work its way into the lamp. Some RTV sealant or regular clear caulk around the perimiter of the housing/lense joint is a good idea.
As Spiffy said, the moisture is more or less harmless. If You've got droplets, then you've got a problem.