We ran into this as well, although we are not using AD I believe the
same scenario applies.

When you switch to forms authentication, there is no default group
provided such as (i.e ntauthority\authenticated users). Thus no way to
permission the default userbase.

We solved this problem by implementing a "role provider" (see MSDN), and
all it does is return a default role for all authenticated users. Then
you can permission this role in SharePoint however you see fit. In
addition, you can further extend the role provider and add more complex
logic to return different roles as well. In our case, the role provider
is only there per the aforementioned userbase problem.

If you want more info let me know. I have this documented.