Windows XP SP 2 (SP 3 is fine) and early Android, I believe, are the clients that don't support certs later than SHA-1.

Lots of old clients still out there, including people who don't have the option to upgrade.

A couple of years ago we tried upgrading our certificate to SHA-2, and rolled it back within an hour, because it broke the site for several of our customers.

It might work now; IE6 users have (finally!) dropped to about 0, but we certainly have tons of IE7 users, and I'll have to look up versions of Windows they're using before we try it again.

We work with hospitals whose IT departments who need to control changes to their computing environments extremely carefully, and upgrades are unfortunately quite expensive and difficult for these kinds of environments.

First, it requires more than just a request flag, since that flag only affects the signature algorithm in your certificate signing request. Your certificate authority has to actually support signing certificates with SHA-2, and also needs a chain that uses SHA-2 signatures. There are some certificate authorities that are lagging behind here, such as RapidSSL.

Second, there are old clients out there that still don't support SHA-2. Namely, pre-SP3 Windows XP and pre-2.3 Android.

Edit: originally this comment said that only IE on pre-SP3 Windows XP was affected; apparently Chrome on pre-SP3 is as well, presumably because it uses some system libraries.