I would sure like to play Texas hold em with RSGP. That would be awesome idea. I doubt i will happen cause many RS users are underage and are not allowed to gamble. The for i don't think its likely to ever happen.
An 'organized casino' would never happen considering the majority of players are kids, 11-15 years old. Just go box staking, essentially the same as Determinate mentioned.
Texas Hold Em in Runescape would be amazing.. Since poker is part skill, part luck.. It wouldn't be the same thing as maxed melee boxing, since that's 95% luck.