IIRC it works for s:selectItems but not for s:convertEntity (it should work for a converter which directly loads entitys based on ids from the page). I used to have some caching support inside s:selectItems but removed it as I found it only marginally useful. If you can come up with a good implementation, I would be very willing to add it to Seam.

s:cache is, really, best suited for output only.