Snap/Grid icon button in 3D view header (edit mesh). FIXME NOTICE: make nice looking icons (yes, that's for you Matt)
When snap is on, a drop down appears for the target method (closest, median, center)
Hotkey to toggle snap is Shift-Tab (this was previously used for a not totally equivalent to Tab. Ton said OK to scrap it). That hotkey is currently restricted to edit mesh (where snap is currently restricted).
Changed a couple of ugliness on how snap settings are stored.