I would like to add a feature request and along with it 10 BTC bounty for this feature: Add an easy to implement hack into Opencart that adds Bitcoin as a currency and gives the user the choice to display prices in Bitcoin or any other currency.
Done. This is already in my latest version. In the admin console upon install, you can choose whether or not to enable BTC as a user-visible currency.
Edit: I also have this available as a stand-alone module, but I haven't open sourced it yet.
Edit: now I have open sourced it and it is available at https://github.com/btcgear/Bitcoin_Exchange_Rate_OpenCart
This code is fully tested and has been in use at http://www.btcgear.com for months.
Please send bounty to 14dTC7bBphANAHzmK9aD7X7Wvo99rhHMTF
