Is it possible to use an iPad as an additional computer monitor?

I just ran across this app from Maxivista that allows you to use your iPad as a second monitor. It's just like their other app that lets you use another computer as an additional monitor, such as a laptop. The problem is, however, it's a bit sluggish as it has to use WiFi to send what is basically a video signal.

So, are there any solutions that allow you to use an iPad as an additional screen via USB?

Recently Air Display announced a beta of the Windows client so now there are two choices but it also works only over Wifi so I would imagine they would both have lag issues.

Duet Display lets you easily turn an iPad or an iPhone into a secondary display. It works with both Mac and Windows computers.