My 26" cruisers that I ride on streets and really tame trails are usually geared in the 57"-64" range. I'm building a kickback bike with rim brakes that I plan to do a lot of trail rides on; that'll have a 49/68" set-up. In the past, I ran 26" and 29" singlespeed MTBs with 50"-52" gears. I think the beauty of a 52" gear is, it's low enough to get up most hills off-road with a lightweight bike (still have to walk some of em), but it's high enough to be worthwhile on long flat stretches and downhills. Going fast on a SS with v-brakes or disc brakes is both fun and reasonably safe; it ain't geared like a regular mtb, but it stops like a regular mtb.
Now, I've been offroad with some c/b cruisers for sure, and it's a different situation entirely. For one, the bikes tend to weigh more than a late-model SS chromoly cruiser--- having a very low ratio helps get a heavy bike up faster, but I really think the temptation of pushing a 50" gear to the max on faster sections would be too great.... and I'd be sure to skid out of control if I had to panic stop while going full-bore on one. No doubt, you'll hit some high speeds descending just from the gravity, but having a higher gear ratio won't really effect that anyway.
I think, if I was going to build a coaster-only bike, purpose-built for trails, I'd shoot for 40"-45".
