Not that I know of. To use a TB iMac in target display (not disk in this case) mode you can only use its TB connection.
While there are alternatives for video (streaming) input (usb and fw) they are not for use in a external monitor (target display mode) configuration.
(My latest wonder is if I can upgrade the video card in the mac pro with a card that has thunderbolt outputs?)
None exist at the present time. Not sure if any will ever appear (that and adapters too).
AS I read this the latest iMac can be used as a display using Mini Displayport:
Use a 27-inch iMac with a Mini DisplayPort as a display
If you have a 27-inch iMac with a Mini DisplayPort, any other Mac with a Mini DisplayPort or Thunderbolt port can use it as a display. Make sure you have a Mini DisplayPort to Mini DisplayPort cable.
I agree with that except the OP said he had an iMac with TB ports, not mini-displayports. While you can connect a TB imac to output to a mini-displayport display it is not bi-directional so you cannot use the TB port for input from a mini-displayport device (e.g., another mini-displayport computer).
That is a subtle distinction, but I think you are right.
Use an iMac with Thunderbolt as a display
If you have an iMac with Thunderbolt, any other Mac with Thunderbolt can use it as a display. The iMac will play both the video and audio from the other Mac.
- Connect the Thunderbolt cable to the Thunderbolt ports on each computer.
- Make sure both Mac computers are turned on and awake.
- Press Command (⌘)-F2 on the keyboard of the external display Mac.
- If you want the external display Mac to play audio as well as video from the primary Mac, choose Apple menu > System Preferences, click Sound, and then click Output. Select the external display Mac from the list of devices.
Right...the Mac Pro that is need of the external display only has minidisplay ports and a DVI output. The new iMac only accepts thunderbolt along side of other traditional ports (usb/firewire). The thing is, I've got this huge iMac screen already set up and I'm using the Mac Pro as a server, with no monitor, keyboard, mouse etc. I typically just remote in to it to use it. However, when you actually need those things for whatever reason (say, reinstalling OS X Lion), you can't use remote and need to hook up a second large monitor which is completely taking the wind out of my sails. I'm desperately trying to find a workaround, even if it means buying 5 adapters to do it.
ScreenRecycler is a solution to your problem. Here's macupdate.com's description:
ScreenRecycler allows you to use your other computer as a second monitor.
Many Mac users have a second computer sitting right next to their main computer on their desk including a nice display. ScreenRecycler now enables you to use this spare computer as second display for your Mac. You can even use an idle machine for that purpose.
It's like a dual monitor setup without the need to buy an additional monitor. Just start ScreenRecycler on your Mac then connect to ScreenRecycler via VNC from your second machine.
Recycle your old Mac and Windows machines !