As far as my non-lawyer view takes me, that line is simply meant to catch the case where you exchange a de-centralized virtual currency for something (anything) else of value that's meant to substitute for currency in some context (i.e. what if you traded BTC for gold? For certificates that represent gold? For another virtual currency like WoW gold?).

That is, FinCEN is saying that you're still a money transmitter if you accept Bitcoins for anything else the Treasury considers to be equivalent to currency (which means, roughly, it can be readily converted back to currency), but not for goods and services. That way, you can't claim not to be subject to the rules just because you don't exchange Bitcoins for dollars.

Remember, the idea here is to make rules that allow FinCEN to track large flows of capital and generally prevent money laundering. So they're concerned with the situation where I give you bitcoins for gold and then turn around and sell the gold for clean dollars.