Make sure you take the time to respond to the employer in a polite and professional manner. The way you respond says a lot about you and your character. It is important to not burn any bridges with a company you might want to work for in the future. See the below for an example of email corresponce with an employer.
Dear [their name],
Thank you for letting me know my offer was
rescinded. Naturally, I am disappointed, but I
understand these are challenging times for
all. I greatly appreciate being selected in the
first place, and I look forward to being in
contact with [company name] in the future.