This may be something that everyone else knows already, but since this tripped me up for a little while, I figured I might as well post a blog about it.
Today, as I was tweaking one of our DataTemplates that was being used in a DataForm in Silverlight 4, I was running into a problem related to the IsEditable property. I had a ComboBox in my template, and I tried everything I could to get it to be read-only (I needed it to be read-only only some of the time - depending on what was set in a different property). After eventually thinking maybe my binding was broken, even though I saw it call the get of my property, I tried hard coding IsEnabled to false. Still to no avail - it kept being editable (much to my chagrin). Eventually, after reading about how DataForms worked, I came up with an idea - I put the IsEnabled binding on the DataField tag instead of on the ComboBox itself. Sure enough, poof! it instantly started working and my ComboBox was enabled or disabled depending on the value that my DataField was bound to.
Hopefully this helps someone who runs into a similar problem. As a sidenote, I also discovered during my testing that you can set something inside a DataForm to be read only by changing the Editable attribute on the property that is being bound to. This wouldn't work in my situation, however, since whether the value was read only or not was dependent on the value of another property, so it made the attribute route more tricky.