This report presented to last weeks 7th International Conference on Mars says the exact site has not been selected but has been narrowed down to 4 general regions:

OVERVIEW OF MARS EXPLORATION PROGRAM 2007 PHOENIX MISSION LANDING SITE
SELECTION.
R. E. Arvidson1, D. Adams2, J. Bandfield3, L. Barge4, J. Barnes5, W. Boynton6, P. Christensen3, J. Friedson2, A. Gillespie7, M. P. Golombek2, J. Guinn2, E. Guinness1, D. M. Kass2, R. Kirk8, A. Knudson1, M. Malin9, A. McEwen6, M. Mellon10, T. Michaels11, A. Mushkin7, J. F. Mustard12, D. Paige13, T. J. Parker2, S. M. Pelkey12, F. Poulet14, S. Rafkin11, J. Rice3, K. Seelos15, F. Seelos15, M. D. Smith16, P. H. Smith6, D. Spencer2, T. Stein1, L. Tamppari2, and D. Tyler5.
Seventh International Conference on Mars.
http://www.lpi.usra.edu/meetings/7thmars2007/pdf/3204.pdf

I'll take a look to see if any of the near-polar volcanoes or gypsum deposits lie within any of these regions.

Bob Clark