I was subleasing a room in a house for four months when I needed to break my lease. I left with more than 30 days written notice, on November 29th (and have paid through December of this year) with a letter stating why I moved out. I also agreed to let him keep my initial $500 deposit. The room is in excellent condition, except for the fact I left my queen-sized bed.
However, I still have two months left on my lease. My landlord/roommate is furious I left and now is demanding I pay him the two months rent I owe him (for January and February) by December 15th, or he'll be forced to take me to small claim's court.
Isn't he obligated to try to mitigate damages by advertising my room and attempting to find a new roommate? Or am I just stuck paying the rest of my lease?

If you signed any kind of (sub)lease agreement, you should read that, and make sure you followed all the rules. You shouldn't've left your bed because he'll likely try to charge you for removal of it, regardless of what it may be worth. Bottom line: He just wants the cash, and no hassle with removing a bed. If you truly didn't cause any damage to the property, your $500 deposit should cover a significant amount of January's rent. Of course your deposit was never intended to be used as rent. Mitigating damages by advertising the room and attempting to find a new renter incurs costs: advertising, cleaning, etc. It also incurs hassle: scheduling interviews and showing the property. All of these things add up to one pissed off ex-roommate. He's stuck paying 100% (instead of 50%) of everything for 2 months, and having to deal with new potential future ex-roommates 2 months early. He may or may not take you to small claims court, depending on any perceived damages (real or imagined). Your 30 days written notice will give you some credibility in court, but the bottom line is whatever it says in whatever (lease) document you signed.

I know he intended to re-renting my room after my lease was up. If I pay through my lease (March 1st) and he finds a new tenant before then, can he legally collect double rent?

No he can't collect from you and a new tenant at the same time. Some states require mitigation and others do not, where are you?

California

No he can't collect from you and a new tenant at the same time. Some states require mitigation and others do not, where are you?

I'm in California

It appears the obligation to mitigate in CA is on your shoulders, makes no senes to me. California Real Estate Property ... - Google Books (http://books.google.com/books?id=8HywIJH4Zc8C&pg=PA232&lpg=PA232&dq=CA+lease+mitigation&source=bl&ots=Q5YBvvjCV-&sig=TEQx3bY_jd45BhZaWn2sb3tLFiM&hl=en&ei=ldD1TL3THoaBlAfJzsWqBg&sa=X&oi=book_result&ct=result&resnum=6&ved=0CCwQ6AEwBQ#v=onepage&q&f=false)