We had put alot of money into our FSA as I was hoping to get pregnant this year - didn't pan out like we wanted so we were left with extra money in our FSA. Our primary care doctor will write a prescription for anything that you ask him for - I haven't tried hard core prescription drugs (not my cup of tea) - but I did ask him to write a prescription for a treadmill desk as a I work from home and sitting all day was killing me. Well he wrote it and I got the letter of medical necessity approved by the FSA administrator submitted the claim and the money just showed up in my account!!
If you work from home - I highly recommend this treadmill desk - LifeSpan Fitness TR1200-DT Treadmill Desktop. You could probably fashion one out of an existing treadmill too - I think I saw a post elsewhere about someone doing that.