I'm using a Macbook Pro with two brand new Dell U2410s side by side. One is hooked up through the Mac's Displayport, the other to a Mac USB port using a Diamond BVU195 adapter and the Displaylink Mac 1.6 beta driver. The graphics card is set to "better graphics" in the Energy Saver preferences panel.
At first I hooked up the display using the Diamond "straight": DVI cable from the monitor to the Diamond's built-in DVI input, output to USB on the Mac.
This caused the display to turn slightly blue, and it seemed to get bluer and bluer over the 2 hours I used it that way. Eventually whites became almost sky blue -- basically unusable. Plus I could not calibrate the monitor's color. I freaked out and found this forum, so I know why I can't calibrate. (Displaylink, please fix this!!!)
Then I hooked up the Diamond to its included DVI to VGA adapter and used a VGA cable from the monitor. Surprisingly, this seems to have solved the problem. Of course I still can't calibrate the monitor, but at least the color balance is more or less normal.