Sega have announced that the 'Supplemental Update' will be coming to the XBox 360 servers sometime this summer.  They have also announced that as a result, online service for the base game without the expansion pack, (which they refer as PSU 1.0) will end on June 21st.  This will not effect offline play for PSU 1.0.  To continue playing, it looks like anyone who doesn't already have it will have to spend the 800 MS points for the expansion.