[bjourne]

You can do it with single bets to. If the following: bookie X offers you odds x, bookie Y offers you odds y, x > y, you are sure bookie Y is a better punter than bookie X ,then betting on odds x is a free bet which will generate profit in the long run.

[iheartmemcache]

In college I did extensive research on this amongst Bodog, 5dimes, and another third party vendor considered 'reliable'. I'd scrape sites hourly because lines move like any market will, and I did this for all 17 weeks. I don't think there was one instance where there was an arbitrage opportunity (my algorithm would go 4 game-pairs deep, so it wasn't an exhaustive analysis but rigorous enough). Even if theres large disparity between bookies, the fact that the industry accepts -115/100 as fair would require a huge mispricing by a bookie for you to arbitrage successfully.

[bjourne]

I'd wager it is harder for sports such as baseball and hockey. But opportunities does happen, at least in football. But ime they are ephemeral so you have to be very quick to catch them. Updating once per hour isn't nearly fast enough. For example if news get out that a player is injured and will miss a game, then the odds will shift but some bookies will be slower to react than others. That can give you a window to arbitrage for 20 minutes or so. You can do it a few times, then the bookies ban you and void all your bets.