Joe, I suspect but am not sure that you have remembered some conversions of R12 systems to R134a going awry. Using R134a in a system that used R12 will cause failure. R134a reacts with the mineral oil in R12 systems and causes it to break down and thus ruins the compressor. Also, any moisture in an R134a system will cause hydroflouric acid to build up and eat away at just about everything (although the gaskets and compressor motor windings are a favorite).
I really hadn't heard of R290 causing any problems in existing well maintained R12 systems. Do you have any urls? I just spent 15 minutes googling, but that doesn't mean squat. I get lost in 100+ hits on the EPA related sites and Autofrost posts.
Good discussion! Learning more each day...
Too bad IRC is still blocked here at work.
BTW:
this is a great site on some of the alternatives to R12.