Early name reservations open Oct 27 through Nov 3 (up to three characters)
Blizzard says pre-purchasers can reserve names for up to three characters total across all rulesets from Oct 27 (1:00 PM PDT in North America, 18:00 CET in Europe) through Nov 3. Names are first come, first served; it requires the Digital Upgrade or Collector's Edition.
Source: Blizzard Entertainment, worldofwarcraft.blizzard.com