The major sites don't do this. We know because many people out there collect literally millions of poker hands observed on these sites and mine the data for every kind of statistic you can think of. If anything significant was out of whack they would have picked it up. Look at the 'online poker' section of the twoplustwo forums for example.

The random number generators used by these sites are hardware systems that use micro fluctuations in ambient temperature (for example) as a source of entropy and they are very careful to use enough bits of entropy for every card shuffled.