I think Jed nailed this one on the head.

Janis,
Potrero Hill is caught in the Eastern Neighborhhods Planning process as we speak. This plan is being worked out to change zoneing to maintain some of the light industry and jobs here is the City. Until that gets finalized, it is my understanding that things are on hold. The only large open space was the S&C Ford facility that was used to do the condo's where the Whole Foods is.
The other land is the current sit of the housing authority properties. The plan there is part of HOPE SF to develop mixed-income housing. Obvbiously in the meantime Mission Bay and the area around UCSF but before Dogpatch is in full swing development wise.
Check the Planning Department web site for information on the Eastern Neighborhoods process.