It's 400 AGL anywhere.
Here is a 2007 FAA document that tells the policy at that time. As has been stated, times are a changing, but this gives a base reference for where the FAA has been coming from.
http://www.faa.gov/about/initiatives/uas/reg/media/frnotice_uas.pdf
"The current FAA policy for UAS operations is that no person may operate a UAS in
the National Airspace System without specific authority. For UAS operating as public
aircraft the authority is the COA, for UAS operating as civil aircraft the authority is special airworthiness certificates, and for model aircraft the authority is AC 91-57."
"AC 91-57 only applies to modelers, and thus specifically excludes its use by persons or companies for business purposes."
ADVISORY
AC 91-57
DATE June 9, 1981
CIRCULAR
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
Washington, D.C.
Subject: MODEL AIRCRAFT OPERATING STANDARDS
L
1. PURPOSE. This advisory circular outlines, and encourages voluntary
compliance with, safety standards for model aircraft operators.
2. BACKGROUND. Modelers, generally, are concerned about safety and do exercise
good judgement when flying model aircraft. However, model.aircraft can
at times pose a hazard to full-scale aircraft in flight and to personsand
property on the surface. Compliance with the following standards will help
reduce the potential for that hazard and create a good neighbor environment
with affected communities and airspace users.
3 0 OPERATING STANDARDS.
a. Select an operating site that is of sufficient distance from populated
areas. The selected site should be away from noise sensitive areas such as
parks, schools, hospitals, churches, etc.
b. Do not operate model aircraft in the presence of spectators until the
aircraft is successfully flight tested and proven airworthy.
CO Do not fly model aircraft higher than 400 feet above the surface.
When flying aircraft within 3 miles of an airport, notify the airport operator,
or when an air traffic facility is located at the airport, notify the control
tower, or flight service station.
d. Give right of way to, and avoid flying in the proximity of, full-scale
aircraft. Use observers to help if possible.