I hate telling you to do this because I was against it before, but you can disconnect your battery for a little while and have it go away. If the gas cap is the problem and you have tightened it, it won't come back on. If it comes back on, you have a problem.

I did this recently, but not to reset the light but to use the battery to test-start another car of mine. When I put the battery back in the Volvo, the light was out and, since I had tightened the gas cap when it first came on, it didn't come back on. Of course, the downside to doing this on purpose is that you have to enter your radio code and all your trip data is erased from memory.