I can't count how many times some stupid browser detection rules caught me as collateral damage - either too new a version (after a new release), or Opera etc. I typically tweet how lame they are, and never go back.
First, if you are going to block IE 8 then you ought to block older Firefox, early chrome, people who have older smartphones (how dare they not buy a new one every 6 months!) and pretty much every release of Safari. And you probably will end up blocking people on Linux or BSD by mistake - or people using secure/solid IE derivative browsers. And everyone on XP that can't run FF or chrome. That's a lot of people to call names and antagonise.
And blocking people on older computers might make sense if you're selling only to the digital crowd, but even then, be careful. You might be surprised what people are using outside the "i have the latest computer to impress my friends" crowd.
If you can't put in the effort to support ie 6, 7, 8 (or opera) then that is fine. Understandable considering some of the hacks on has to do, the knowledge it requires and all the testing effort. But you don't have to be an ass about it and cast judgment on people and rub their noses in it.
And if it is to put some of the annoying new features of modern web design that make a site hard to use, navigate and impossible to bookmark, well then, I have no sympathy for you.
Websites ought to be designed for their users, not used for their designer to compete in the cool leagues of featuritis...
oops, soapbox alert. getting off