I have something like that set up in my house, but I also have a motion sensor so that it comes on when it's dark and I walk by.
Parts for your project:
light sensor $1 U.S.
arduino $30 U.S.
light switch - I use a powerswitchtail, $20 U.S. (120 volts)
or you could wire up your own relay, make sure it's waterproof, and do some research here so you don't shock yourself.
powerswitchtail: Information.com People Search | Free People Finder & White Pages - Locate Anyone
light sensor tutorial: How to use photocells, LDRs, CdS cells, photoresistors!