Shadowstep is by far the most fun Rogue ability in the game. I remember back in the BC days when Subtlety was viable for PvP, and Shadowsteps made heads roll. Sadly, those days are gone, along with Subtlety's viability in PvP, much less PvE.
That's why Shadowstep should be a core ability. It's not just because it's a fun ability that any Rogue, regardless of spec, will find useful in PvP (and in some cases, PvE); it's because it just fits the whole overall mindset of a Rogue. Shadowstep is currently in the "Subtlety" tree, a tree with talents that are silent and deadly, and thus ninja-like. Yet Shadowstep can also fit the "Assassination" and "Combat" trees: appearing spontaneously behind the enemy can lead to a quick and efficient takedown, emblematic of the assassin; and in the heat of combat, teleporting to an enemy's back can give you the upper hand immediately through the element of surprise. In game, Shadowstep is used in all of these ways due to its low cooldown and ability to get a Rogue out of a sticky situation, like a snare from a Druid or Mage (along with CoS), and turn the tables.
Blizzard, if you do anything to drastically change Rogue talents for Cataclysm, make this at the top of your list. Shadowstep is the most fun Rogue ability ever, and was the envy and despair of all classes in PvP. Let all Rogues use it...