PlayOn has been collecting some interesting data on PvP in World of Warcraft. And while I'm not sure any of the information provides grounds for conclusion, it's certainly interesting to peek at. Both of these charts take into account sample data from 128,477 unique characters across 5 realms (one RP, two PvE, and two PvP) during one week in October. Above is their chart of average PvP rank for each class -- as you see, shaman takes the top spot and priest the lowest, but there's relatively little difference in rank between the two. (Perhaps suggesting that any class can be competitive with effort? Or simply that more shamans, warriors, and mages PvP?)
Then we have average PvP rank for time played. Unsuprisingly, those attaining rank 13 are nearing 80 hours a week of playtime (for reference, the Daedalus Project suggests the average MMO player spends 20 hours a week in game), while those at rank 14 are dropping closer to 60 hours (possibly a sign of no further play efforts after reaching the maximum rank).