Halo 2 for Vista supporters will need to wait just a bit longer to "continue" the fight. Apparently, there were some problems with the DVD discs (physical problems). The new release date is May 31, 2007.
UPDATE: There were rumors circulating the internet that H2 Vista would be delayed till July 4th, but these are bogus per Microsoft spokesperson. May 31 is the date.
UPDATE: I swung by the local gamestop last night and H2 Vista is on scheduled for June 1st.