The airline I was flying for at the time was fully approved for enroute navigation, in MNPS airspace, with two Honeywell HT-9100 GPS units as the sole means of navigation....and this was in 1998.
Type aircraft, Lockheed L1011.
Now, I expect others will argue otherwise, however...they would be wrong, at least in our case.
Can't say about other airlines specifics, however.