According to Blizzard, the streaming trial client will be temporary unavailable while they prepare the upcoming "Echoes of Doom" content update.

Interested players can still try out WoW for free by downloading the full trial client here: PC, MAC. The streaming trial client will be up and running again a few days after the launch of the patch.

My guess is that most of you have full accounts (or maybe even two or three), but this is still good to know. The full Blizzard post from the WoW official site can be found below.

The streaming trial client, which allows trial users fast access to the trial version of World of Warcraft, will be temporarily unavailable while we prepare for our next major content update, "Echoes of Doom." Trial users can continue playing World of Warcraft by downloading the full trial client here: [PC ] [MAC ]. Streaming trials will be brought back online within a few days of Echoes of Doom’s release, and we will provide further updates as they become available. Please visit our Technical Support forum for additional information.

You can upgrade your account to a full version at any time. Just purchase and install the retail version of the game (available at most software and video game retailers), then enter the provided Authentication Key at our online Account Management service to upgrade your account. After you upgrade you will still be able to play the character you create during this trial. For details about the trial version client, refer to our Trial Account FAQ.