I just finished building up my Gary Fisher Rig (singlespeed 29er) as a fixed gear, so I think I'll take this one. I have it set up with a rigid steel fork (Redline), a BB7 disc brake in the front (borrowed from another bike - I have an XT with an 8" rotor for it that needs a bleed), 165mm cranks, 39x17 gearing, and 2.2" tires.

In your case, I think it's a great idea. Mine is a lot of fun for around town riding (which is why I built it), and will make a perfect errand bike. Mountain bikes also have plenty of room for racks and fenders. I'm not having much trouble keeping up with downtown traffic (even with the 2.2" knobbies, which are on their way out). Clearance for really big tires is also a big plus, especially if it will see a little off-road use. Track bikes on the street are a lot of fun for dashing around, but an upright fixed cruiser is a whole different animal. Mine feels (and looks) like a humongous BMX bike. I can't speak much to skidding (not really my thing), but if you find hard tires and use a ratio with enough skid spots you will be fine.

Go for it.