Refueling is typically done in standard refueling "tracks" where the tanker flies around a large, four-cornered "race-course" shaped trajectory in the sky whilst thirsting aircraft line up to be "serviced."
It makes no sense for this to be done over a populated area, or at low altitude, except in some unusual emergency situation.
I am in the Air Force (5 years to retirement!) and witnessed the very thing that Psalm37 witnessed over Houston on my way into work at Ellington Field. I thought the very same thing at first..."who are they intercepting?". They were flying west and low (about 5000') and banking towards to south. Of course...I have a source of info that Psalm37 doesn't...my office is right outside the Wing CO's office. So...they first thing I did was ask him and he told me it was CAP being refueled.
BTW...the guys flying CAP now over NYC and sometimes DC are from Houston...the same unit that escorted Bush back from FL on Sept 11th.