Transmitting HDMI/DVI over an FPGA with no support for TMDS
Good questions. I have been researching this topic myself recently, and will try to provide some brief answers here.


  what exactly is TMDS vs LDVS?


LVDS (low-voltage differential signaling) is simply an electrical specification for a differential signaling interface, while TMDS (transition-minimized differential signaling) is both an electrical specification AND a specific 8b/10b encoding scheme. Unfortunately, the electrical specifications are not directly compatible, although there are various ways to "adapt" one to the other under certain circumstances.

Electrically, LVDS uses totem-pole drivers and differential termination (100-120&Omega;), while TMDS uses CML (current-mode logic) open-collector drivers and individual 50&Omega; terminations to +3.3V. The Spartan-3E IOBs do not directly support CML.

Then there's the question of TMDS coding. The Spartan-3E IOBs have support for DDR, with data rates up to 628 Mb/s, but no dedicated high-speed SERDES logic. You would have to do the TMDS encoding and decoding in the FPGA fabric, using the DDR support in the IOBs to get the final bitstreams. This would limit you to pixel rates of 62.8 Mp/s or less.


  In a practical sense, I am wondering if I can simply wire an HDMI breakout board directly to the input pins of my FPGA, configure those pins to the LVDS IO standard and expect it to work


No, not on the input side, at least not without some effort to terminate the TMDS properly and then AC-couple it with proper bias to the LVDS receivers on the FPGA (all of this while maintaining an accurate 100&Omega; differential impedance). Note that the sample projects you link to are all output-only examples. Driving DVI/HDMI from an LVDS output seems to be much more forgiving; they don't seem to have added any bias or termination resistors to their PCBs.

Your best bet would be to use external DVI/HDMI input and output chips, and make the connection to the FPGA via their parallel buses. I have used Analog Devices parts in the past.