I've read a number of articles saying the better brokerages are taking more controlled risk with their own capital to maximize their PnLs. GS comes to mind.
IB has shown their prowess with their marketmaking abilities.
Trading gain as a percentage of total revenue.
2002: total revenue: 599, trading gains: 472, percent: 78%
2003: total revenue: 651, trading gains: 488, percent: 74%
2004: total revenue: 621, trading gains: 423, percent: 68%
2005: total revenue: 1099, trading gains: 640, percent: 58%
2006: total revenue: 1736, trading gains: 805, percent: 46%
It appears IB has become less reliant on their trading to grow revenues. But it also shows that they seem to be very good at it.