If the slit is in the sidewall, absolutely... replace the tire. If it's in the tread, and you're wanting to eke out a few more miles, put one of the long, rectangular vulcanized patches over the hole from the inside of the tire (like a boot), then a strip of duct tape over that for a little security.

I booted one of the Vittorias on my road bike like that yesterday when a piece of glass punched an annoying little hole through the tread. I'll keep an eye on it for awhile while I look for a matching tire... It's only got about 2700 miles on it so far... ;-)