Live poker is tax free. Holland Casinos are owned by the government.
Online poker is officially illegal, but because the government doesn't have an online casino of their own, like Sweden and Austria, it is sort of allowed, as long as you pay taxes on your winnings.
Online poker (and gambling) winnings are taxed on a monthly basis at 29%. No carry-over of losses allowed.
Winnings include bonuses and rakeback, losses can be deducted. Related costs like buying a laptop can not.
So you better make sure every month is a winning month.
When you live 6 months+1 day in the Netherlands in the year, you're obligated to pay these taxes, and report this yourself. If not, you can count on getting fined for failing to submit a timely tax return, once they track you down.
There are a handful of people working for the Dutch tax office that are scouring the Internet full time, looking for the people behind the Holland-based screen names they see in the PokerStars tournament results and forum posts.
Bank transfers from eWallets and ATM withdrawals with Neteller cards are monitored. There is no escape
A large group of poker players are currently fighting this in court, in some sort of class action. It can take a few more years before this ends up at the Dutch supreme court, where a final ruling on the matter will be made.
It's expected that online poker will be regulated before then. The government planned on handing out online gaming licences this year, but the recent elections delayed that process.
It should happen in 2013.
Companies that are expected to apply for a licence are PokerStars, party.bwin, Unibet, Everest, Betfair, Ladbrokes, Bet365, and William Hill.
Have fun