single Mic for acoustic gigs

http://blog.shure.com/how-to-mic-a-bluegrass-band/

I found this and thought it was good reading.

I have never done this, so this is more thinking out loud. I would probably default to a condenser used only for recording (to prevent any feedback loops), and a cardioid pattern (facing the band with the dead zone to the audience, if possible). Omni/Figure-8 would just be asking for trouble IMO (especially with "unknowns" in the picture). If using only one mic, you may also see a situation of having to position instruments for best results.
 
As tlw mentioned, many more factors come into play here, especially if you are limited to what/where you can place things.

The question is a little outside my envelope, too, but I know that the Shure KSM-32 has been a standard at the Grand Old Opry for years. I've seen it used often for acoustic instruments, on everything from pianos to mandolins.
 
The photo in that piece is a KSM-44, but it's essentially a more expensive multi-pattern version of the KSM-32.
 
I wouldn't think an omni mike would be very practical (bleed and feedback problems), unless you wanted to do the old-school thing with multiple singers or instrumentalists gathered around one mike, Beach Boys style. However, the KSM-44 would give you the option of omni, cardiod or figure-8, plus it has two high-pass filter settings the KSM-32 doesn't have. It's also physically larger than the KSM-32, making it more obtrusive visually.
 
All in all, I think a KSM-32 might be a safe bet for miking individual instruments.

Ditto on Johnny V's comments. I have a pal in an acoustic bluegrass band that play/sing around one AT4047 or a 4033, on a restaurant patio (off Lake Michigan) and when the wind blows you don't hear them or maybe only half of them. They like it, but in the audience it's very "iffy". When it's quiet they sound good, otherwise they are sort of lost to the noisey crowd, and when the wind blows ....forget it. It's all wind noise!
Gigging from '60s acoustically sucked! I used Barcus Berry when those came out, but they sucked too! LOL
In the '80s I used an EMG Strat pickup in the soundhole of my Guild F50 plus a pair of BB inside on the plate. I had a jack for the EMG going to an echo and a 15" JBL, while the BB's(on jack 2) went to a chorus and then the PA. That sounded really good once I got the EQ/balance worked out. Kathy Mattea once walked in front of me and stopped dead, saying, " wow! your guitar sounds great, like a grand piano!" and we talked about the rig on my break. I thought it sounded great too. LOL I got the rhythmic strum and lots of sustain-y bottom. I recorded off the PA and several engineer pals would always listen and say "there's 2 guitars! Who's playing the other one?" The affect of the piezo and the EMG captured 2 completely different flavors.
       Nowadays, there are way more options, though one mic really depends on the stage setting and then the band has to learn to work it, too.
Michael

Our group uses two Shure Super 55 microphones (http://www.musiciansfrien...5-dynamic-microphone). This is a dynamic microphone, but still picks up good. There are 5 of us in the group - all acoustic except for the bass. The mic works very well, although you sometimes have to work your instrument (for instance we have to lift up the mandolin a bit and move to the front of the mic when doing a solo). In addition to a good sound, it looks good too if you like the old style microphones!