What I would like to do is to disable all fields (put them into "read-only" mode) if a field's value is "Submitted". Is this possible? What might the solution look like? Thanks!
In InfoPath2010, you can add a conditional formatting rule that changes a section to disabled, which means read-only. You can then copy this rule to other sections... easy to do.
You could also copy your view into another view - and mark the view read only - then switch to that view if the field = submitted in a form load rule.