There are a couple of things you would need to consider/scope out.
- What API are you going to rely on for prices? (What backup api are you going to use if the first is not available)
- How often are you going to update the BTC to Currency?
- Real time on page load? (possible performance issues)
- Cron running every few minutes? (possibly losing out on BTC/money with old data)
Once you've figured that out it's a case of building a payment method module, there are a lot of good resources out there, below are a couple of good ones:
Personally i wouldn't go with the real time/page load as you are depending on the API's loading quickly for your pages to load quickly. I would use a cron and save the current BTC price in the database maybe every 10-15 minutes. Then you can retrieve that value of 1 BTC from the database and do the math against the quote total.
Wherever your bitcoin wallet is would need to have an API too that you could hook into to confirm the payment has been sent (unless this is going to be done manually) You would probably need to ask the use for confirmation on the address they are using to send.
With all that being said, there is a Magento extension (I've not tested or tried it) that is an add-on to the Stripe payment gateway that allows you to accept bitcoin as a payment: Cryozonic But you would need to check the code to make sure it's not doing some malicious or stealing the bitcoins.
The Bitpay option that @BBQ mentioned looks like a good option too.