Platanos- I'd reset the computer and see if the fault light comes back. One of the articles that I read earlier said the first thing to make sure of is that it is not a glitch. So if you reset it and it never comes back, don't worry about it. If you reset it and it comes right back I'd take it to a mechanic.
I replaced the O2 sensor on my Ti well after 76k miles, but it had a lot of higway driving. If you don't drive the car on the highway very much or do a lot of short trips where the car cannot get up to normal operating temperature deposits build up on the O2 sensor and it will eventually not be able to work properly. Sometimes you can just go and run the car up on the highway for 30+ minutes and it clears the crap off of the O2 sensor.
Can it hurt the car? Yeah, it could. I think some folks on here have said that running really lean can cause some serious issues. You will probably also get really bad gas mileage.
Here is some good info on how the O2 sensor works, and when it should be replaced and why it fails.
http://www.autohausaz.com/bmw-auto-p...n-sensors.html