I'm trying to use the ComboBox for use with auto-completion in a grid and hit this problem in IE. Luckily, I could reproduce the problem in your test code.

1) run test_edit_dijit.html in an IE 7 browser
2) select "Toggle singleClickEdit"
3) click in the priority cell (it works fine, you can change it values and when you remove all test, possible values show)
4) select "Add Row"
5) click in the priority cell, can't set focus in this cell or in any other cell in the Priority column