With today's release of the HiRISE image of the Mars Pathfinder landing site, it's interesting to speculate about HiRISE locating Phoenix on the martian surface, assuming the landing is successful.
The candidate landing sites appear to be relatively featureless, which is typical of those northern latitudes, so triangulation by taking bearings on horizon features from Phoenix SSI imagery will be difficult, to say the least. MARDI descent imagery will be critical since, presumably, Phoenix should pass over something identifiable in orbital imagery. And remember how MER DIMES images helped zero in on the Spirit landing site.
I'm not sure of the details of the Phoenix EDL sequence but I'm assuming that some orbital asset, perhaps MRO, will monitor its telemetry as, for example, MGS MR did for MER. And I believe MRO Electra has the ability to get a better position fix on radio signals than either MOC MR or Odyssey UHF.