If Google had stuck with the standard OpenID implementation, they could've still had the same user experience. http://zain@gmail.com/ is a valid URL, and Google could've used that as the OpenID URL.
It seems most OpenID implementations handle identifiers without the http:// prepended.
If the libraries don't already support the "@" syntax, they should.
If Google's way of doing things requires any proprietary changes, I'd say it's unacceptable, but if that's the only change necessary then it seems like a reasonable way of doing things.