Currently WFS is slightly inefficient in that it asks for a BBOX depending on where the user is looking making it next to impossible to cache effectively. This patch introduces tiled WFS, so it only asks for WFS along grid boundaries.
This patch effectivly works, there is a demo here:
http://tile.openstreetmap.nl/testwfs.html
I started making some tests but ran out of time. Also, it got decided the have a single WFS layer with singleLayer:true/false. I beleive this works but havn't tested it entirely.
There's more discussion about it here:
http://openlayers.org/pipermail/dev/2008-February/002378.html