The only thing I've had SNI fail under (so far) had been Netscape Navigator 3.0 and at that point, does it really matter?
IE on Win XP. Or at least anything using the built in crypto stuff. I think firefox will still use it's own. Not sure about chrome.
You might be talking about HTTP 1.1 Host header which allows vhosts for plaintext HTTP servers. SNI allows this to happen with TLS (HTTPS, etc.) servers.