Thanks for all your input! I replaced it with an Armadillo with reflective sidewalls. Now I have obviously mismatched tires, but fortunately I don't really care that much.

I'll be honest: I really did want an excuse to try this reflective sidewall thing out, and I've heard good things about Armadillos. Handy! But I didn't get the slash on purpose, and I would've probably just lived with it, except I found that when I rode to work on Friday, the knowledge of that cut in my tire made me feel nervous and unsafe the whole time. Since I rarely take risks I can avoid, I replaced the tire that night before the bulk of my commute.

Now seeing all the struggle the LBS people had getting it on -- they have thumbs of steel and they needed a tool to get it on, and even that was a struggle -- I'm not convinced I'll ever get it off to fix any flats. Hopefully I don't get many!