[Gonso]

I can't speak for poker sites' RNG, but I know in the slot industry (I work for WMS, and prior to that worked for IGT), RNGs can be figured out - BUT, if you can get that far, then there are also drastically easier ways to compromise a given machine (such as replacing an EEPROM chip or something).

My point is, with technology in it's current state, the RNGs are virtually the least vulnerable security risk in the process.

Also, not all RNGs are software-based. You can use geiger counters, or even atmospheric noise.