i want to make a field required only if the value of another field is "foo". is this possible in Infopath 2010?
You can do that with a workaround.
Use a custom button to submit the form and run set of rules when the button is clicked, i.e. a validate rule and a submit rule. Set the validate rule to run if Field1 is "foo" and Field2 is empty. The action is to set a trigger field that shows/hides a section with an error message. Stop rule execution after the validate rule, so the form won't be submitted.