As of last year, the FAA had no rule about this and I still don't think there is. There is a guideline under one of the AC's to stay below 400', but it's not a rule or law. That's not to say things have changed in 2010. I was actively reading the discussions about all this on rcgroups last year and the beginning of this year.
EDIT: Here's a link to the pdf on the AMA's website. This was from last year and was the purposed rules. If anything past, it would have been in the last two weeks, as there is a current thread on rcuniverse on this.
http://www.modelaircraft.org/faa/recommendations.pdf
If nothing else, I'll throw a mode C transponder in there, set to 1200.. Problem solved.
FAA AC from 1981:
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/1acfc3f689769a56862569e70077c9cc/$FILE/ATTBJMAC/ac91-57.pdf