also, if you use fix-a-flat type of stuff, be sure to forewarn your mechanic when you go in to mount the new tire. when my customers forewarned me and i put down rags on my tire changing equipment, the surcharge was only $15. if they didn't warn me, it was $42. i hate when people try to use that crap. it never works and really is messy to clean up. there are still marks on my Coats 220 from some a-hole's fix-a-flat slime that i didn't clean up quickly enough. :-[
anyhow, good luck, and the leak is pretty quick. it is unsafe to ride at any speed with less than like 30 psi in the tire. what happens is that the tire heats up a lot with the loss of pressure and can "blow out". while i would be surprised to see that happen at 27 psi, one would want the best safety...
I've had nail-holes plugged before... if it's in the center of the tread-wear, then it's probably pluggable. If it's closer to the sides of the tread, get a new tire. Cycle Gear sells "tire insurance" where they will replace your tire if you run over nails, etc (just prorate it based on how much tread you have left). If you get insurance, then just replace it.
check with your local Cycle Gear shop... found a pair of Macadams for $200 total... guess the price isn't too bad (could be better maybe?) but I thought it was a bargain since the retail is about $50 - $70 more (for both)