According to Hearthstonecast from July 2013
Matches in “The Arena” mode follow a match making protocol to ensure that players of similar play strength (meaning players with comparable win/loss records) are matched up against each other. Obviously, such a system would improve over time and not be as reliable in the early stages of Hearthstone going live.
The link to HearthstoneCast is broken right now, but this has been confirmed by Blizzard devs on twitter several times. The arena matchmaking algorithm does not look at your previous arena or constructed history and it does not consider the card quality of the deck you have drafted.
Current (unconfirmed, fan-based) thinking is each match you are matched to someone with the same Arena score as you, so for your first game you are playing someone who is also on 0-0, if you then lose, your next game will be against someone who has also just lost one game.
I would guess as well that matchmaking is something that will be being fine-tuned for a while.