I guess I have to weigh in on this one. As I have said on other boards, I have been asked, IFR, if I have portable GPS on board by ATC. I was filed /U. The controller (Chicago area) asked ME what heading I needed to fly to whatever fix. I told him what it would take, he approved it, I flew it as an assigned heading off airway (legal!!!!). As said previously, it's the RAIM that precludes the use of a portable in IFR.