I have a legend toc setup almost just like the ESRI sample using the new legend control in the 2.1 api (which is GREAT btw)... when I check layers on, they seem to pretty much always turn on, but when I check a layer off, a good majority of the time the map does not refresh this data away and the progress bar just hangs at like 80-90%... I can then just do a little pan on the map and the data loads right in then.
With 2.0 I built my own TOC control very much like this templated legend control... and it had the same behavior... so that is why I don't know if it is the legend control itself, but if something else is going on.
Fiddler never shows an attempt to connect to the ArcGIS REST endpoint either... I was thinking before maybe it was a caching issue and 2.1 would maybe fix this issue, but it didn't. My application is a WPF XBAP by the way.
An observation I have made: this issue does not ever seem to happen with the first layer that is turned on in the service, but it does with the layers folowing it... especially with layers in group layers. I can turn the first layer on and off pretty much everytime without this issue. *EDIT* After trying this some more, maybe no so much even the first layer, but if it is the only layer it always works (but a trip to the server map not be needed for that).
Hope that made sense... any ideas?