I’ve just installed the plugin on WordPress 3.1 and while the core functionality seems to work, some of the back end editing seems to be broken.
Most noticeably when creating/editing widgets the AJAX bits are somewhat hit-and-miss if they will respond or not. Sometimes work on the first click, sometimes won’t work until after a few clicks, sometimes requires a page refresh.
If you need any more specific information on this please let me know.
http://wordpress.org/extend/plugins/mycustomwidget/