Best Buy's weekly circular is advertising Call of Duty 4 for $44.99 this week, both online and in stores. Unfortunately, that's only for the PlayStation 3 version and not the Xbox 360 version.
Not to be outdone, Amazon is also selling Call of Duty 4 PS3(affiliate link) for $44.99, which will save you over two bucks in sales tax. 360 owners are, again, out of luck.
I'm still not sure if I'll get off my duff and pick it up, but I'm highly tempted.