No there is not a DVI, Displayport, or even a VGA to Thunderbolt adapter available yet.
It looks like if such a device were to be made it would have to have a DisplayPort input, to work, or include active convertors for other legacy signals because DisplayPort is the integrated video signal with Thunderbolt.
To do this a DisplayPort to Thunderbolt adapter must be possible first, hence requiring a full Thunderbolt Controller with a Display port input. Since a device like this has not been built yet, but technically should be possible according to the double ended arrow in the diagram for the DisplayPort signal on the external controller, it should be possible for such a device to exist.
If an when built this would provide a true Thunderbolt signal including a DisplayPort signal and could meet Apples requirements for Target Display Mode on Thunderbolt equipped iMacs, however there is a gray area as highlighted below.
iMac (21.5-inch, Mid 2011) and iMac (27-inch, Mid 2011) support Target Display Mode via Thunderbolt to Thunderbolt cable (2 m) when the source is another Thunderbolt-equipped computer.
So how enforcing Apple is between the terms Thunderbolt-equipped computer and Thunderbolt-device would also factor in if this will work or not...
So at this point we are waiting to see if a Thunderbolt to DisplayPort device is possible, and if that device would be able to send a signal that a thunderbolt equipped iMac would accept while in Target Display Mode.
At this point its just to early to tell if adapters will be available. So yes avoid Thunderbolt iMacs if you want to use Display Port - Target Display Mode, or buy a external Display to share between your PC and Mac.
I wold say external display for the Win, Win case!!!