In my opinion, no. But I've been known to be wrong.

IE8 is the newest browser available for Windows XP which still makes up a vast number of business computers.

When they say "Not supported" what do they mean?

If it gets the mobile style then that's not so bad. If it all goes a bit Picasso then that's a problem.

this. I just started a new project in Foundation 3 for a public sector NFP that requires IE8 compatibility. Foundation 4 is pretty much not an option for me as a result. Guess I'm stuck with Foundation 3 for now.