Front wheel oscillation
It seems like it could be:


Loose Hub
Loose Headset
Untrue Wheel
Tire messed up one way or another


From your description, it's hard to believe that the source is in the rear half of the bike (I'm assuming it also happens when you're not pedaling, correct me if I'm wrong).

It seems to be a fairly minor problem (for now at least), that could be very hard to pinpoint.

If you can't find the problem after a technical inspection, I would just keep riding. If you notice the problem getting worse, inspect it again (it'll be easier to find).