"HMRC, the UK government's tax-colllecting body, says people do not incur a tax liability when trading in bitcoins as long as they do not turn their profits into regular cash. Once conventional money is involved, however, the income could become taxable, HMRC told New Scientist"
That's good to know and what I thought was the case. It's probably best to check with your local tax accountant, though, since how this is handled exactly might be different in different countries. It also might change over time. Right now, I think Bitcoins can be considered a commodity, in case Bitcoins become an actual currency, things might change.
At the moment, I think buying Bitcoins is kind of like buying credits with some services (like Fotolia, for instance); but the more there is a "Bitcoin-based economy", the more countries will try to directly tax Bitcoin transactions.
I think registering with New Scientist and writing friendly letters to the editor might be a good idea :-)