EVE typically has a winter expansion and a summer expansion. The winter expansion, Quantum Rise, is just days away. The summer expansion is expected (but not confirmed) to include Walking in Stations, only there's a little twist with the timing... They're not waiting for summer. It's coming in March. In a box. In your local game store. That's the information we have at this time, but we'll keep you informed as more details surface.
Update: Note that we haven't locked down a release date with Walking in Stations yet. Massively will report on Walking in Station's official release date once that's available -- at this time we only know it's tentatively slated for 2009.