This topic has been mentioned before (primarily summer 2011) but it's a good warning to those who bet there.
Betfair claims this effects 0.5% of all users but please note this is likely to effect anyone who makes money/withdraws money from BetFair.
How betfair (exchange site) works. There is no juice in the listed odds. You can either lay a bet (be the bookmaker) or back a bet (be the bettor). Let's say you back a bet: $100 bet to win $100 (ev odds). Your bet wins. However, betfair takes commission of 5%. So, only $95 is credited to your account balance. If the bet were to lose, you simply lose the $100.
Betfair has something called Premium Charges (PC) that effect about 0.5%. PC comes into effect when/if your gross winnings are too high compared to the amount of commission that you have paid. The magic ratio of gross winnings to commission is 20%
Betfair imposes this 20% tax after you have bet in 250 markets. jjgold and roxxxyfish clear 250 markets a month. If you only bet 1 game a day on average you'll clear 250 markets in less than 9 months.
Situation, you have bet in 250 markets and have won $10,000 but only paid $1000 in commission. The 20% tax comes into effect. It is removed (if necessary) every Sunday at midnight. The first week you make $1000 but only pay $120 in commission. Well 20% of $1000 is $200. Since you have only paid $120 in commission, betfair will remove another $80 from your account balance.
There are also other premium charges that betfair says only effect 0.1% of all users. These are 40%, 50% and 60% Premium Charges on your weekly winnings. The 60% comes into effect if your gross winnings to commission ratio is lower than 10%.
If anyone has any questions or wants clarification please ask. I have been reading about this for the past day as I will soon fall into the 20% category if I don't change my betting styles. I am not pleased.