I think it's a pedantic point, as I believe that language is as language does.... however, there's no reason why it couldn't be like "gold", which is also highly divisible. You need the "s" plural for things that can't be divided (like dollar bills). Thus, we don't get paid in "golds".

That said, semantically, coins aren't usually thought of as divisible things in a modern context-- they are pluralized, so bitcoin will probably be pluralized in the same way.

nwh has a point though. Bitcoins aren't things. It's not just that they don't have physical form. For example, you can record the serial number from a dollar bill and find it again later. You can't do that with "a bitcoin" because there is no bitcoin-thing that can be tracked, even digitally speaking. Each address has a balance, and each transaction deals with amounts of bitcoin. There is no owning or transferring of identifiable "bitcoins".