Best interview I ever had was when a company I faxed my Resume called and said "Hi my name is .... I'm calling from, .... I wanted to let you know your hired. When can you come in and negotiate starting salary?"
That was a good day as I had been out of work for 6 months searching for a job.
Don't read to much into this kind of question. Most of the time, unless you are completely unqualified, they will ask this question. If in reality you can start that day, that is the answer. They might need someone now and that could be the deciding factor. If you need to give notice at your old job, tell them that. They will appreciate that you are not the kind of employee that is just going to disappear without notice.