USD conversion rate API I want to build an mobile application which tells you how much your bitcoins are worth in USD. I would like to know of good API that I can rely on for data. As well as historic conversion rates over time. 

Example. 

I have 5 bitcoins -> which are worth 5031 USD. 

Bonus points for other alt-currencies included in said API. 

I would like something that isn't linked to a particular service or exchange.
It will never be accurate unless you know the preferred exchange for your user, because as you can see the prices are different for each exchange.
Go to this website http://bitcoin-analytics.com, and click on "Arbitrage", you will see the difference in exchange rates from multiple sources.

Your best bet is to ask the user to choose their exchange or if they have a preferred one, and use their API, which means you'll need to use multiple APIs for most popular exchanges, like mtgox.com, bitstamp.net, btc-e.com..etc, I would also consider which geographical area you are targeting, it would help you identify who are the ones you need to have.

Good luck.