(found at http://www.worldofwarcraft.com/patchnotes/patch-05-04-06.html )

* The /pvp flag has been changed to a toggle. Now once /pvp is turned on, you will be flagged for PvP until it is turned off. The /pvp flag will remain even after logging out, so please remember to turn off the /pvp flag if you do not wish to engage in PvP. This option is only available to players on Normal and Role Play realms.
* Item effects that increase the chance of a critical hit with spells will now work properly with healing spells.
* Fixed a bug with Cho'Rush the Observer where he wasn't properly contributing to the tribute loot mechanic in the North wing of Dire Maul.
* Ranged attacks will remain as they are on the live realms - unable to be dodged or parried.

Rogues
* Ambush, Backstab, Ghostly Strike, and Riposte now properly increase the attack power contribution by the percent modifier. Our goal is to make the dagger-based Backstab/Ambush build a viable alternative to the sword/mace-based Sinister Strike build. As a side effect, fixing this bug also increased maximum critical Ambush damage beyond our intentions. After considering a variety of balance changes (including reducing the percent modifier or base damage of Ambush, or reducing the Improved Ambush talents) we decided to change the Lethality talent to no longer affect Ambush. This does not counteract the DPS increase from the bug fix as Backstab is the primary source of DPS while Ambush is a source of high, one-time damage and cannot be used as consistent DPS. After all balance changes are considered, here are some details:

(Using a Barman Shanker with 2.0 weapon speed and between 500 and 800 attack power)
* Ambush without the Lethality bonus and attack power bug fix (patch 1.4.0) causes between 4-12% more damage than - - - * Ambush with the Lethality bonus and attack power bug (patch 1.3.1).
* Backstab (patch 1.4.0) causes between 9-13% more damage than Backstab (1.3.1).

Mage
* Cone of Cold will not be subject to diminishing returns.

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Defense will apply to ranged attacks (against other players).

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In 1.3.x /pvp just turns on a voluntary PvP flag that automatically wears off after five minutes.
In 1.4.x, /pvp remains on until toggled again, at which point it will wear off after five minutes.

If you wanted a macro that had the same effect as the old /pvp command, it would contain this:
/pvp
/pvp