There are plenty of cases, though not a huge number in comparison to the number of I-864's submitted by sponsoring spouses. You can find lots of them with a well aimed Google search. Just to get you started, try searching for...
Stump v. Stump, 2005
Cheshire v. Cheshire, 2006
Shumye v. Felleke, 2008
Yoonis v. Farooqi, 2009
Those are specific cases, but a set of carefully selected search terms should bring up lots of articles and other information. Try something like...
I-864 divorce enforcement "support order"
...or some similar group of relevant terms.
I recently had a PM conversation with a member who is beiing threatened by their spouse's divorce lawyer with an I-864 enforcement action. In summary, I told that member that it depended a lot on the state, the county, the court, and the judge. If someone should find themselves on the receiving end of an I-864 enforcement threat then, at the very least, they should make sure their attorney is intimately familiar with the federal law and precedent cases in their jurisdiction. Also, don't ignore the possibility that a failure to get an enforcement order in a state's family court does not necessarily stop the I-864 beneficiary from seeking enforcement in a federal court. The Rooker-Feldman doctrine generally prohibits a federal court from reviewing a state court's divorce orders. However, if the beneficiary is not directly seeking to overturn a finding in the state's court (e.g., the state court refused to hear the case citing lack of jurisdiction) then the federal court may accept the case. In that event, you need an attorney with experience in the federal court system, and your divorce attorney may not be up to the task.
In short, it's not common, but it does happen.
