If you haven't merged your World of Warcraft account with a Battle.net account yet, you better do it soon! As a bonus incentive, players who do merge their accounts will receive Oswald the penguin! Those who already have merged their accounts need not worry as they will also get Oswald. Players that were involved with the Wrath beta may remember receiving one of these in the mail except he was known as Mr. Chilly then. I'm sure Oswald will make an excellent companion to Pengu (another penguin pet). Receiving Oswald is going to be a limited time offer. Blizzard hasn't officially announced a date yet, but they do plan on removing the ability to get him at some point.
Of course, not being allowed access into the game might be incentive enough to begin the account merging process!
For additional information, check out the Battle.net account FAQ, Instructions on creating a Battle.net account, a video tutorial on how to convert your account, or just make your account a Battle.net account now (US) (EU).
Note: There is a blue post saying November 12th, and the in-game announcement saying November 11th. If appears that EU players need to convert by the 12th, and NA/US players need to convert by the 11th. But be on the safe side and just convert your account now.