It is account based.
Your Windows Phone Marketplaces purchases are tracked the same as Xbox Marketplace purchases. Your Purchase History is available online, same as on xbox.com.
Purchased games can easily be re-downloaded to a different phone as long as your LIVE account is on it. Game progress / save data however is another story.
Also, I don't believe the same thing applies to other Marketplace items like music. I had a bad download and there was no way to download again. I ended up just getting a refund, but I now use Amazon for music DLs instead of Zune.