Is it possible to fire a worlfow rule when formula field updates?

Workflow doesn't fire on formula fields, as the field isn't stored in the database and so doesn't get updated. What happens is when you retrieve the record, the value is calculated and the field populated.

There's a couple of ways around this:

(1) Don't use a formula field, but use a regular field and a workflow field update that replicates the formula.  That way the field will be stored in the database.

(2) Replicate the formula in your workflow rule and fire it every time a record is created or edited.