Basically RevA allows for faster upload/download speeds. The advantage is faster network performance.
RevA is active on the network, the 6800 (dont know about other models) handset is expecting a firmware upgrade Q1 2008 to enable both RevA and AGPS
Taken from wikipedia:
http://en.wikipedia.org/wiki/Evolution-Data_Optimized
There have been several revisions of the standard, named alphabetically starting with the first as Rev. A ("revision A") while the first standard is referred to simply as Rev. 0.
The EV-DO feature of CDMA2000 networks provides access to mobile devices with air interface speeds of up to 2.4 Mbit/s with Rev. 0 and up to 3.1 Mbit/s with Rev. A.