Since PvE to PvP transfer option became available, I've kept a close eye on my own realm to see if there'd be any discernible change to transfer patterns both on and off. As far as I can tell -- no. I've seen a few people transfer off but there doesn't seem to be any more so than usual. Only a few players did so purely for the purpose of PvPing on a different battlegroup, and most of them, like Zach Yonzon, had previously been PvP-realm transfers or rerolls anyway.
The flood of high-level PvE to PvP transfers gloomily predicted by many doesn't seem to have materialized. People who'd rather play on a PvE realm are doing just that, and most of the people who'd rather play on a PvP realm were already there (never saw that one coming, eh?). I thought at the time that Blizzard's decision would probably have its biggest impact, not on PvP players, but on PvE raiders who had previously been restricted to recruiting PvP-to-PvP, PvE-to-PvE or (much less commonly) PvP-to-PvE only. If you've ever been in a raiding guild which found great recruits from the "wrong" kind of realm, you're probably familiar with what a headache that was.
I play on a medium population PvE realm that launched when Burning Crusade hit. Since we're not really at the cutting edge of either PvE or PvP content as a result, mine is probably a bad sample size as we're not a hotbed of transfers either way. So, I'm not sure if my own experience is representative. If you're playing on a medium-to-high population realm, either PvE or PvP, are you noticing any difference to the traffic patterns, on or off? If you're on a low-population realm, has your situation improved or worsened?