Well since most of your PvP will be pugging in the battlegrounds (assuming), then you'll have a hard time making use of a shaman's strengths. I have a level 62 shaman.
Shamans lack a lot of their most useful moves at 19. (Ghost Wolf, Chain Heal, Chain Lightening, Most Buff Totems)
Their main dps moves are Earth Shock, Lightening Bolt and Frost Shock. The shocks are amazing for burst damage and interrupting casters. Lightening bolt is really not powerful enough for the long cast time, too slow in my opinion for fast paced PvP. With dps, the problem is you have nothing to do when your shock is on cooldown. Cast Lightening Bolt? You opponent will start running in circles on top of you, interrupt, or run out of range. With all the swarms of hunters running around, even getting into melee range can be a challenge.
Totems. Like shocks, they have a pretty small range. Their damage is average, and thy can be quickly destroyed. Useless? Absolutely not, but a bit of luck, timing, and skill is needed. Earthbind Totem is the best totem in terms of pvp usefullness, but avoidable if your opponent knows what they're doing.
Lets talk healing. Cure Poison and Purge. Great abilities! A sneaky shaman can be incredibly annoying with Purge. Next are lesser and greater Healing Wave. Pretty basic, with decent numbers. Not having a shielding move or instant cast will make PvP Healing much more difficult though. Because every other healing class has these moves as well, they're not really the best healers. Doable? I've seen amazing shaman healers. Skill can make up for a lot of weaknesses.
I've concluded that shamans are best for defense because of their totems, and are great for breaking through turtles with Purge and Earth Shock interrupts. A priest or druid will be probably a little better for heals, and rogues and hunters will be better damage. So not the best, but definitely brings some great utility to the table in the hands of a skilled player.