The problem is the implementation. What the driver does, is plug in between the card and the OS.

Tricks the card into thinking you have another DVI somewhere, tricks the OS into thinking the card has just grown a new DVI.

The problem with the implementation is that it is not scaleable. And... you constantly depend on either the OS or a new driver (from the real video card) to not break things... This does not happen very often.

What they need to do is create a driver that plugs in a NEW VIRTUAL VIDEO CARD from scratch. Then the problems with the cards goes away, and the OS already knows how to handle multiple gpus.

I just bought a Toshiba Dynadock at a reduced price, seems people are RETURNING them like hotcakes.

What they need to do is create a driver that plugs in a NEW VIRTUAL VIDEO CARD from scratch. Then the problems with the cards goes away, and the OS already knows how to handle multiple gpus. But this means they actually need to write a real display driver AND "virtually" support DirectX and others.
So doing this is NOT cheap... essentially the driver is kind of a "hack", a good one. It works as it should, most of the time, as long as you don't push the limits.

dual graphics card with displaylink usb & windows 7
 

I am having problems with my arkview displaylink usb adapter on windows 7 64 bit. At one point I got it to work, but it failed after rebooting the system and it has not worked since. Multiple uninstalls and reinstalls have failed to fix the issue. Things I have tried:

run as administrator
compatability mode - windows vista
arkview drivers 5.2 10/27/09
displaylink drivers 5.2.22663 12/08/2009

I have the following fatal error in my event viewer:

Product: VB Installer Installation -- Error 1720. There is a problem with this Windows Installer package. A script required for this install to complete could not be run. Contact your support personnel or package vendor. Custom action SMSI_Parse_String_P1_185 script error -2146827859, Microsoft VBScript runtime error: ActiveX component can't create object: 'SMSIConvert.SMSICustom' Line 1828, Column 1,

Even though this event is logged it seems as though the install completes successfully but my monitor does not get activated and the led light on the adapter does not light up. If I unplug the adapter and plug it in again windows "detects" the device and "installs" the appropriate driver, but it doesn't work. The adapter was working on windows vista for a while and I am convinced that this is a software issue.

Btw I have a dual graphics card.

Widows device manager recognizes that there is an adapter installed:
DisplayLink Display Adapter (8060)
DisplayLink Corp.
12/8/2009
5.2.22617.0
This device is working properly.

Ok, managed to get it working again, here is what finally worked:
After many attemps of unplugging and replugging the usb cable to the adapter(windows recognized and installs the device driver etc, but video did not work, led light did not glow). This is what worked:

Then I unplugged the adapter, and disconnected the dvi cable from the monitor.
Then plug in the USB cable with the DVI disconnected.
Allow windows to recognize the device and install the driver, led light on adapter lights up!
Now attach the DVI cable to the monitor, windows recognizes the new display and extends my desktop, Yay!

There must be a bug somewhere(displaylink/microsoft) that doesnt allow the adapter and monitor to be recognized at the same time when you first attach the device. Stay tuned to see if I need to redo this dance upon reboot.

It may be named "DisplayLink Display Adapter " but windows sees it under USB devices.

Dual 4870's with CrossFire ENABLED + Displaylink = WORKS Perfectly!
 

A little late but just wanted to leave this info in case someone stumbles on this thread using Google.

I can confirm that on the following setup multi-monitors using DisplayLink WORKS perfectly!

Laptop:
Dual ATI 4870's with CrossFire ENABLED
GPU Drivers: ATI's 10.5 beta
Intel Core i7 Quad processor
4GB Ram
DisplayLink Drivers: 5.3

The usb monitors work flawlessly. Video playback is perfect. No slowdown. All bells and whistles from Operating System available. ATI Catalyst Control Center also works perfectly. No issues in gaming on main monitor, or any deterioration of performance detected. Ran 3DMark Vantage and 3DMark06. Scores are similar with and without USB monitor enabled.

All works great.