My impression is that the GPU on the current Server model doesn't support 3 monitors.
I had read that the current high-end Mini could drive 2x Thunderbolt displays, plus an HDMI.
Does anyone know if the current high-end Mini could drive a DVI or Displayport monitor hanging off a Thunderbolt display?
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Yes, the Server model can't, but this years models will be able to do so - probably even the ones with the Intel HD 4000 chip.
Yes, you can use a DVI or DisplayPort monitor at the end of the Thunderbolt chain. However, it's technically not possible to connect it directly to a TB display, so you have to chain in another Thunderbolt product.
However, there's still the HDMI port, which can drive at least a DVI display (not a DisplayPort one), and maybe we'll see two TB ports on the high-end Mini (although I don't think so).