I believe I have found a bug in my installation of Access 2010: When I set the Ribbon property ON MY STARTUP FORM it is ignored. However, if I set it on any subsequently opened form, it works just fine, and shows the relevant ribbon.
Perhaps it is because the AutoExec makro, where I load the ribbon is not the first thing executed?
Anyway, the workaround is making a splash form with a timer that opens the "real" startup form.
Jock