If you must. If you have shopped your little heart out and have not found pancetta, have not ordered it online, and can't bring yourself to brave the grocery store once more, or the grocery store absolutely does not have it and you've asked the manager and he said he absolutely will not stock it, or you live so far from the store as to make it impractical to make a special trip, then yes, you may substitute bacon for pancetta.

Because bacon is smoked and pancetta is not, you might want to blanch the bacon in boiling water for two minutes to cut the smoke flavor as much as possible before adding it to your dish.

David Joachim, author of The Food Substitutions Bible suggests in the following order these substitutions for pancetta: guanciale (cured hog jowl), American slab bacon (blanched), Canadian bacon (adds smoke), salt pork, prosciutto, smoked ham, and smoked sausage.

He also notes that the Spanish tocino and the French ventrèche are quite similar to pancetta, but, as your stocks of pancetta are unacceptably low, quite likely you have a shortage of tocino, ventrèche, and cured hog jowl in your pantry, too. Pity....