Four-year-old World of Warcraft seems to have matured enough now to allow the transfer of PvE characters to PvP servers. "[..] We don't feel that PvE-to-PvP transfers will have the negative impact on the game that we initially wanted to avoid when we started the service in 2006." Unfortunately, the transfer service is not free at all and weighs, for each paid character transfer, a hefty US$25.00 (plus taxes), or 20€ / £15 in Europe.

For the full Paid Character Transfer FAQ WoWarians in the States can have a look here, whereas Europeans are welcomed here.

Apparently, far too many players (for Blizzard to cope with) have taken the opportunity to transfer their characters now. This suddenly increased the player population on the PvP server's from low to medium. The swamped Paid Transfer Service is thus down at the moment but should be back up ... sometime.