Well, it turns out that HFC-134a isn't as good a solution as hoped for. As such, the refrigerant has been banned in Europe beginning in 2011 and it seems likely the United States will follow suit. Enter HFO-1234yf, also known as 2,3,3,3-Tetrafluoropropene.
The new refrigerant was co-developed by DuPont and Honeywell and has already been approved for use in Japan and Europe for automotive air conditioners. Earlier this week, the United States EPA issued a proposal to approve a replacement for HFC-134a, and this new stuff is seen as a likely substitute.
[Source: EPA via Green Car Advisor]