is it possible to run just one cog out back (and ditch the mech) and run a double upfront?
Really? Why don't you think about that for a few seconds and see if you can answer your own question.
You'll need way of tensioning the chain to take up slack due to the small front ring but yeah you could.
The rear much will be needed if you have more than one gear, front or rear, due to changing chain tension
Truvativ Hammerschmidt?
Just buy two bikes and get yourself a bike caddy to follow you around
You could run an Alfine chain tensioner.
If both front rings were the same size, then yes.
Not quite, but have you considered dinglespeed?
Are you American?
if you could work out 2 sprockets and 2 front rings that need the same chain length it could work and give you a wider range. ie smaller front ring with larger rear and larger front with smaller rear.
ahh that' a dinglespeed.
[url= http://www.schlumpf.ch/hp/schlumpf/antriebe_engl.htm ]schlumpf[/url]
I got one 
yes it works but u need to run the mech (just use a cut short cable to get tension) i ran a 16 in the rear and 32 44 up front just running just a left shifter,worked really well and had the ratios i wanted,just remember to set chain length on the bigger sprocket i ballsed up first time setting on the 32 to short
If both front rings were the same size, then yes.
You would still need some slack to derail the chain
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It could be done using horizontal dropouts but the shifts are slow.
A dinglespeed is probably what you want: two chainrings and two sprockets. So long as the sum of the teeth is the same for each pair, it should work with minimal changes to the axle position.
I've thought about running 36:14 for riding to the trails and 32:18 for once I'm there, but decided to just run 32:17 and pedal faster/harder an appropriate.
You could run a double chainring with a single sprocket, provided you used a rear mech as a tensioner, but if you're going to do that you may as well run a cassette and change gears with a shifter as it's all the drawbacks of gears without any of the benefits.
Or you could do something like this when your in the smaller ring...............[img] 
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ok the ghost chainring blew my mind for a bit! it's quite cunning.
Theres a Dialled Alpine on the clasifieds that the guy is running 1 x 3.
Where the 1 is at the back.... 😕
http://singletrackworld.com/forum/topic/dialled-bikes-alpine-full-bike-pikes-hopemavic-wheels-etc
Seems like an excellent way to get all the disadvantages of rear gears, in a package without the gears?
Orange had a prototype P7 setup as singlespeed but with a Hammerschidt at the front which seemed quite a neat solution to me.
Some googling suggests they tried it on a Sub zero as well: http://singletrackworld.com/forum/topic/fs-unique-strangeorange-sub-zero-frame-with-hammerschmidt-600
That floating chainring thing looks genius, even if running true singlespeed isn't that a better solution than a tensioner on a conventional dropout frame? Or in practice does it move/fall out?
Really? Why don't you think about that for a few seconds and see if you can answer your own question.
Harsh.
To the OP, the answer is yes. I converted my old Lava Dome in this way. Two rings up from, singlespeed at back with a chain tensioner. Got the chain length just right and the tensioner had enough range to allow me to use both rings up front. With original front mech 'thumbie' shifter, it was bombproof.
Great set up, allowed for the hassle free aspect of singlespeed but meant I could ride up steeper stuff as well.
May well convert it back to this tonight, I only took the mech off for a clean and never got round to putting it back on!
[img] 
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Yes, I've done it, I used an old mech as a tensioner and had it way stretched out. If your canny you can even kick the chain off onto the lower chainring without stopping.
Pic dosen't really show the set up too well but you get the idea.
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Why did I do it? The bike was a singlespeed and I fitted a larger front ring to use on road sections of a mini tour. I'd already used the rear mech as a tensioner.
I considered this last year using a triple with the rear mech as a tensioner.
Why?
Because in winter 90% of maintenace is about shifting issues, cassette wear etc. Front mech can bully through the muck, rear mech is a far more sensitive beast.