Is it worth converting 'normal' currency into Bitcoins with the hope of making a profit later, by converting the Bitcoins back into 'normal' currency? In other words, could it be seen in a similar way to investing on the stock market? You take a risk, but potentially you could profit if the value of Bitcoins increases with respect to (say) the US Dollar? And, if so, are there people already doing this kind of speculation?
There are certainly people doing it. Anyone who holds Bitcoins who isn't moving them around is speculating in this sense. And even if you move Bitcoins around, if you don't move them sooner, you're speculating at least a bit.
As for whether it makes sense, that's a tricky question. There's always a risk Bitcoins could take a nose dive in value and never recover, say because they were made obsolete by a superior crypto-currency or impractical through governmental action.
In my personal opinion, it's a pretty good risk for money you can afford to lose. (Higher than average risk, but much higher than average possible rate of return.) But you may have to make some tough decisions when the price drops: Do I sell now, take a small loss but at least cut my losses? Or do I risk losing everything if the price never goes up? And if I do sell, and then price starts going up, when do I buy? What if I buy and then it nosedives
See What are some of the risks of speculating on Bitcoin value? for a discussion of some of the risks involved. It's a bit more focused on active trading than just holding, but there's a lot of overlap.
Ultimately it is a gamble. General consensus holds that as Bitcoin is more widely adopted and spent on goods / services; the exchange rate will rise.
Operating a store that accepts Bitcoins, and/or spending Bitcoins will help that exchange rate go up.
Yes, it could be seen in a similar way as investing in the Forex market. And like the Forex market, you can't predict with a high degree of confidence what the price will be in the future.
People are certainly doing this kind of speculation already - take a look at the speculation section of the bitcointalk forum. There are also some charting tools to predict trends such as SierraChart, BTC Trader or simply Bitcoincharts. One bitcointalk user (S3053) is even selling weekly in-depth technical analysis of bitcoin prices.
Personally, I also think Bitcoins are a high risk investment right now. There are too many uncertainties about its adoption as a currency. You can find many questions here at stack exchange about its weaknesses and undefined legal status and there are many more news across the web. All these factors can influence bitcoins price both short-term and long term.
For the next 14 months or so, until block 210,000 bitcoins are being issued at the rate of 7,200 per day roughly. At current levels, the rate that the currency is being inflated is at about 36% on an annual basis. At current exchange rates ($8.40 USD), the value of that is about $60,000 per day of currency issued. Here's a post describing that with a little more detail: http://www.bitcoinmoney.com/post/8627223073
If there is more demand (either economic transactions using bitcoins or hoarding by miners and speculators) than this $60K then the price will rise. If there is less than this $60K the price will drop.
But after block 210,000 if the exchange rate is still $8.40 at that time, then only $30K per day will be issued.
Now this is an already known fact and it may already be priced into the currency (i.e., it might be trading at less than it is now if that weren't already known).
The bigger unknown is how much currency will be used in commerce at that time. Will it just grow incrementally by tiny gains, or will adoption some day explode like some speculators and Bitcoin kool-aid drinkers (such as myself) believe will happen. If it does, those who bought bitcoins at $8.40 will be seen as the "lucky early adopters", much like those who asked the same question as you when the BTC/USD was about $0.30.