First, I've done my due diligence to read the postings on this forum as well several other websites on lease laws. I understand that I am liable for damages to the landlord as a result of breaking the lease. What I can't find is what happens after the landlord finds another tenant and no damage was realistically done?...
On July 22, 2009, I signed a lease to rent an apartment starting on Sept 1, 2009. I gave the landlord a check for last month's rent and a security deposit (equivalent to a half month's rent). He stated he would need to run a credit check first before he would sign the lease. I told him that's fine but I would need to know the results of that credit check soon because I need to inform my current landlord whether or not I will be renewing my current lease. I waited a week where I did not hear back from him so I looked at another unit in which the landlord did not need a credit check. I committed to that landlord instead.
A day later, on July 30th the initial landlord finally got back to me on the credit check and said that it was OK'd and he signed the lease. I informed him right away that I had committed to another unit because I did not hear back from him. He proceeded to say we both signed the lease (and therefore legally bound) and he will to keep the check that was for last months rent and security deposit as violation of the lease. On Aug 6th, 7 days later, he informed me he found another tenant and signed a new lease with him.
My question is - is this landlord still entitled to any of the check I gave him (last months rent and security deposit)? From what I interpreted on the laws, I do not I owe him any of that money because 1) He did perform his good faith attempt to find another tenant and successfully found one to move in on or before the start of my original move-in date. Therefore, there are no damages to him in the form of losing on a months rent and 2) the security deposit I gave him is for physical damage to the unit or unpaid rent. There is no physical damage done to the unit because I haven't moved in yet. There is no unpaid rent because he's found another tenant to fill that time period I had leased for.
There was also no initial agreement on a deposit to hold the unit while he was doing a credit check.
I can also put a stop payment on that check that I gave him because he has not cashed it yet.

Editor's Response
There may be some nominal damages spelled out in the lease, such the cost of running an ad or running a credit check, but that should not amount to much. I would send a letter by certified mail demanding the money back. If the landlord does not comply, sue him in Housing Court or Small Claims Court. Both are very friendly to non-lawyers. Good luck.