State Farm classifies it as a wagon. So does the state of Maryland.
I went through this issue with the Maryland MVA when they informed me that our 5 required standard passenger plates, instead of the "M" plates that adorn SUVs and minivans in Maryland. In fact, one time when I called, I was informed that an internal memo dated March 28, 2006, specifically stated the Mazda5 was a passenger car and hence required standard plates. They were pretty adamant about it.
I have no idea what Maryland is calling 5s now, but the majority of them around here do sport standard plates.