Great effort has been put into Bitcoin's concept of money supply and that there will never be more than 21 million units.
However, it's relatively easy to fork it and create alternate block chains. SolidCoin, IXCoin, IOCoin etc are much less worth, but they can easily be traded against Bitcoin. Does this increase the over-all supply of Bitcoin-like money to well beyond 21 million units? Some believe yes, and that this is one reason already for the recent price drop.
Also, the concept of open and decentralized P2P money is still young. Chances are that a lot of other such currencies will pop up in time. Is there a risk that Bitcoin will be thwarted by an inflation of other currencies, rendering it and all other open, "free", decentralized money worthless and useless?
EDIT: One thing to consider in this context is that the number of traditional currencies will always be limited as there are only so many nation states on this planet (and they are consolidating, e.g. Euro), but the number of decentralized digital currencies is potentially unlimited.