I have done some playing around with this, and i think that the edit form web part is the thing that is actually creating the ribbon. If you close or delete it, then the ribbon does not display.
Also if you try to do the same on the NewItem page you get this message:
You are about to permanently delete
this Web Part. Deleting this Web Part
will render this form unusable. You
will not be able to re-insert this Web
Part using the browser. Are you sure
you want to do this?
And then this when you try to create items
Unable to find the default new form
for list Test.
If you have a look at:
You will see the code that displays the ribbon. This is the edit form web part. I guess you could copy it in your custom web part to make the ribbon display.