ACFS stands for ASM Cluster filesystem, which is basically a cluster filesystem on top of ASM Diskgroups. It is similar to DBFS but kind of more elegant.
Until this momen it was not available on Exadata, but now ACFS is officially supported on Exadata from 220.127.116.11 onwards. To utilize this feature both your Grid Infrastructure and RDBMS home should be atleast on 18.104.22.168 on Exadata.
I just created a test ACFS filesystem on my X4-2 Quater Rack. And its amazing !