I doesn't look like set_visible(true) works.  I tried the latest release as well as the current beta.
    
                                set_visible(false) seems to work fine.
I have the following code:
| var txt = $find('<%= pnl.Items[0].FindControl( "txt" ).ClientID %>'); | 
| if(cb.checked) { | 
| txt.set_visible(true); | 
| } else { | 
| txt.set_visible(false); | 
| } | 
pnl is a radpanelbar.  txt is a radtextbox.
