GridView update inside UpdatePanel hangs

I have a GridView inside an UpdatePanel. This GridView has a command
field (edit, update, cancel). When I click edit I go into update
mode, refreshing only the GridView, as you would expect. When I click
cancel or update the browser hangs.

Stepping through the debugger I can see that the update code is
executed when update is clicked. As far as I can tell, the server
side code completes running. This is just a presentation problem.
The browser then locks up and I cannot click any control on the page.

Thanks,
Don