Depends who you talk to...I've come across it several times, but most of the time it was duplicate therapy and one was d/c'd when I brought it to the physician's attention. There is a proposed mechanism justifying combined ACEI/ARB...
But like no-see-um mentioned, there have been studies that look into combo treatment...clinically there didn't seem to be much benefit. Here's a decent article that discusses a few of the studies (specifically for HTN):
Combination ACE inhibitors and angiotensin II receptor blockers for hypertension. Finnegan PM, The Annals Of Pharmacotherapy, 2003 Jun; Vol. 37 (6), pp. 886-9.