this might help a bit:
FAASTeam - FAA Safety Team - Learning Center Library Contents
here's a quote from the above:
Perhaps this summary will make it easier. Every IFR-certified and installed GPS unit allows the pilot to descend to LNAV (or Straight-in) and circling approaches. Baro-VNAV-equipped GPS systems can also descend to LNAV/VNAV minima. WAAS receivers can descend to LNAV, LNAV/VNAV, and LPV minima. Need another hint? Look for the DA designation versus the Minimum Descent Altitude (MDA) abbreviation on the minima line. Only procedures with vertical guidance have DAs. A descent angle may be provided on procedures which have only LNAV minima, to aid in a stabilized descent, but the MDA must still be respected.