I am currently running a single chainring up front with an 8spd cassette in the rear, and no front derailuer or chain guide. However, the chain does fall off every now and then - primarily when I am quickly droping gears and go over a bump/pothole. I would recommend using a front der. to prevent this from happening (I'm just too lazy to grab one from the parts bin and put it on...). One tip to minimize the chain from falling off is to use the the shortest chain you can (and still shift gears) so that you can maintain chain tension.