In TBC, shadow was an even worse spec than it is now for PvP. It was back in
vanilla WoW that it was truly good.
Now, since 3.0 some improvements have been made. Namely, Dispersion, Improved Shadowform, and to a lesser extent Psychic Horror.
As a shadowpriest, you will be nowhere near the top of the game for PvP, and most just respec disc (still generally regarded as THE priest PvP spec).
That's not to say that Blizzard isn't paying attention to us, however. Holy is getting some love, and shadow is getting one or two
much needed changes itself.
However, if you do intend to PvP as shadow, then I would highly recommend taking a look at
the PvP section of Shadowpriest.com.
There you can find what builds and glyphs to use, and advice on what to do when in arena and facing certain builds, ect.
I would also recommend
Alla's PvP forum/Theo's "special" place.
Good luck melting faces!