Of course that's the concern. Along with the fact that some of these people won't be able to pay their tax bill cause they've blown it all on the internet.
The only report I have seen today is of Tessa Jowell criticizing the US online gambling ban, calling it the new prohibition. This statement would seem slightly at odds with the report you appear to have seen flip. Additionally, she is calling for Online Gambling companies to register in the UK to act as a "hallmark of quality". Now that move is almost certainly to obtain tax revenue. http://news.bbc.co.uk/1/hi/uk_politics/6100012 .stm
I know 2 people who no longer work but earn from interne gambling. I think the government are definately worried about taxes. I think this because they are also with the other hand wanting to open up giant casinos? Is that what I heard?