Has anyone tried to run a single chainring up front. If so what was your gear combo?

I have run a Campy drivetrain with a 42 chainring and a 13-26 cogset. I worked just fine. The only difficulty was finding a left hand non-ergo brake lever that was inexpensive and matched the Campy ergo shape. This is not a problem if you already run bar end shifters. At the end of the day, I switched back to a double chainring up front. The extra gears were nice to have on flatter courses with longer tarmac sections.

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I definitely recommend a single chain ring over a double for CX. I starting using this set up at the end of the 2003 season after having numerous dropped chain and chain suck issues when using a double. Since then I've been using a 40t chain ring with a 9spd 12-25 or 12-27 cassette depending on the course. I've never had any problems since and my results have improved. It saves from making stupid shifting mistakes. In my case, I continued to use my Ultegra shifter/brake lever instead of looking for a brake lever with the same feel.

on my cc-bikes i use 42x 12-25 or 11-23, depends on the track. i ride campagnolo 10 speed. when i rode shimano, i had 12-27, but i didn't need the 27 one single time. because the 12 cog is necessary, i don't use 13-26, and take a 12-26 and a 13-26 cassette and make a 12-26 out of it is way too expensive(considering that i need at least 6 pairs of wheels at the races...)
i love racing with a single chainring setup, you don't have to think abuot it during the race, and when i had 39-46, i always rode on 46, to fix the chain better. so i could no more ride with high cadence(lance-style...)
and on this homepage it is important to say that single chainring is lighter(i use spooky carbon disks left and right of the chainring).
for my right brake lever, i took a campagnolo-lever and took away the "shifting mechanism", this is lighter than the time trial lever you can buy at campa.
my friend rides shimano, he took a 7800-lever, took away the shifting parts of it, this is over 50 gr. lighter(those things don't make sense if you have to buy the parts, but if you get it from the sponsor, make it and get your bike lighter!)

SRAM makes a 12-26 cassette that is very inexpensive and somewhat light weight. Great for cross, they last a long time and are very easy to swap as they are one piece.