Two FAA references covering the legal use of iPads for use in flight planning and flight are:

InFO (Information for Operators) 11011, dtd 5/13/11. Subj: The Apple iPad and Other Suitable Tablet Computing Devices as Electronic Flight Bags (EFB)

AC 91-78, July 2007, Use of Class 1 or Class 2 Electronic Flight Bag (EFB)

A third reference is an article entitled "Can I (Legally) Use My iPad?" from the FAA Flight Standards Service on Page 21 of the September/October 2011 issue of FAA Safety Briefing.