Oh god, not this shit again.
Look at any typical SLA and tell me you don't see a zillion loopholes for lawyers to wriggle around. There's never any real guarantee of service unless an ironclad SLA is hammered out, and I suspect that if Google gets traction with Gmail for Domains, they'll do it.
When I was at Google (2003-2004) we had something like 10 seconds of user-visible outage. That was before Gmail, but it was also with a staff of ~200 people and ~250K servers. I have every reason to believe that Google can still stomp the shit out of all contenders on uptime; they just need some incentive to do it for specific services.
(my $0.02 only; I don't work for Google anymore. In fact I doubt I could ever go back; it would be too depressing to see what's become of the bullpen atmosphere in ops. Even still -- Urs may be the best in the world at what he does.)