A slot machine casino approved but unbuilt at Dania Jai-Alai is too risky a bet -- for now. A spokesman for Boyd Gaming, owner of the fronton, announced Wednesday that South Florida market conditions -- including a ''prohibitively high'' state tax rate, the continued poor performance of three other Broward casinos and competition from Seminole Indian casinos -- mean the company will indefinitely postpone plans to develop a slot-machine casino at the jai-alai property.