You don't invest in money, whether it is fiat currency or bitcoin currency. You invest in things that are known to have long term desirable value. You may speculate on currency fluctuations (and futures, and so on), you might even do well out of it, but that can only work while there is liquidity in the system, while transactions are being made.
For bitcoin to have long term desirable value, it has to have a purpose. It's purpose is to make transactions, at the moment specifically catering to a desire for anonymity. However, in order for there to be transactions, there has to be both a buyer and a seller, and this can only happen if the currency is stable. In unstable currencies, all transactions cease (because either buyers or sellers stop trading, depending on the direction of the instability).
At the moment the bitcoin currency is relatively stable (allowing for the small size of the economy), as mining replaces the bitcoins that have been squirrelled away into 'investment' accounts. As mining becomes exponentially more difficult, the deflation will also increase exponentially. Initially, that will simply increase the value of a bitcoin. At some point though, either people will start to bail out when they see the end in sight (which happens when there aren't enough sellers in the system), or the satoshi will no longer be small enough...
There's no point in having a million dollars worth of bitcoins if you can't spend them on something you actually need or want.
Don't get me wrong, I like the idea of a peer based financial system, and I watch the bitcoin experiment with interest. It has technical merit, but I am not convinced of its long term stability. And stability is the key factor in a sustainable trade system.