Would a normal ethernet cable survive the conditions in an air conditioning vent?
Outdoor rated cat5/6 has temperature ranges from -5 to -15 F to 160-170 F.  I know that I have seen cable ran through commercial ducts.  Do I know if it passed local inspection?  No.  

That being said a home duct would come no where close to 150 degrees F - not talking about duct work from ovens.  I guess you would worry about the casing becoming soft and sticking to the duct.  I don't see the fire hazard though.  

Basically the issue is - is this code or law abiding where you live?  In the US that is a no.  I don't know about other countries though.  

I was just answering the question asked.  Not trying to start a controversy - would I run it in my home through ducts?  No.  If I did I would use a variant of fireproof cat5.