that's true about the atlona 200, but the dp400 does support a higher resolution for the 27 inch monitor, i have been researching this same problem for weeks now. my video card does have a mini display port, its an asus 78970 ddr5 2 gig card, and i do think that the dp 400 and the apple cinema 27 inch could work with this card, i know it does not on the apple TB monitor, but they do hook straight-up from mini to mini display port, but nothing shows. its not the connector in this case, its that its not thunderbolt io enabled. si i figured that why spend the extra cash trying to make it work when there are other really nice IPS monitors around, like the new dell ultra 27 inch, even has a better response time of 6 ms compared to 12 ms on the apple TB and cinema. its cost just about the same, around 900 or so dollars, so i just gave in a bought the dell instead. there are many manufacturers heading into the mini display port arena , so i am sure if we wait a few months, there will be others besides the ati 5500 i think does have the mini display too, i was very surprised to find out the asus 7870 radeon ddr5 had the mini display port, 2 dvi, hdmi as well. so to sum this up, i would just give it some time, get another ips monitor, there are a few really nice monitors out there, the dell ultra seems to be the best at the moment, but that changes ever month. sorry i could not be of much help, but i thought i would share my experience with the very same problem...good luck poster!
scadarick
It has been confirmed that the Atlona converter will not correctly convert a DVI/HDMI signal to DisplayPort for use with the iMac 27" display input. Speculation is that the 1920x1200 max resolution is the problem. I don't know why the iMac display won't recognize and switch to that resolution, but then I don't know why it does not have a DVI/HDMI port in the first place.
Belkin is rumored to be releasing a converter that with both convert the signal to DisplayPort and scale it to the correct 2560x1400 resolution but it will likely cost $150-$200 - and who knows what (if any) performance hit will obtain using a converter box.
Right now it seems the best way to use an iMac as a display for your PC is to buy a DisplayPort capable card. Thanks Apple!