lander,
There is certainly collusion happening at the higher limits but the good sites are doing a lot to minimize it. You get protection from the sites monitoring IP addresses, doing stats on how often pairs of players are on the same table etc. and if something looks out of line, they investigate it. If they make a blatantly suspicious play, you can report it and if it turns out it happens regularly with those players involved, you will likely get reimbursed by the site and the players who did it to you will get banned.
Of course these measures just reduce collusion, not eliminate it. Still, I would say collusion hurts you less than tokes to dealers and cocktail waitresses and you get way more hands online. This is only my opinion and not a hard fact so let the player beware!
What is fact, though, is that a good buddy of mine does very well at 20-40 and higher limits and he has never cheated in his life.