It just smacks of the same thing we saw with the whole go dark for ie thing. Elitism....or making the world a better place :-)
Normally I would consider this to be another case of browser bigotry (and you know how I hate that!), but we are talking about Google Apps and not some smug elitist web designer that just doesn't feel like fixing his CSS to look right in some older versions of IE.
There is an issue of what is and isn't supported by some versions of IE...some newer things like HTML5, which is not purely aesthetic. If Google has plans on incorporating more HTML5 stuff into their Google Apps, things not supported by IE8 and older, then it could make perfect sense not to support browsers in which these things will not work.
And IE8 has hit a stage where no new features will be added, only security updates, so this will not change. What IE8 supports is pretty much set in stone at this point.
And while they won't be cutting off IE8 users from using Google Apps (yet), the message displayed telling them to upgrade their browser, I think should be more accurately stated as "Buy a new OS, or perhaps a new computer."