Forum OpenACS Q&A: Response to Postgres, distributed databases, two phase commits, or other replication strategies....
I'm not sure about whether they implement two-phase commit ontop of databases which don't support it. I'd have to defer to an app-server wonk.
There are several companies that are rolling out akamai-like distributed caching specifically for dynamic content. I thought persistence's Dynamai was one of them... These seemed like a much cheaper way to maintain uptime than managing a global db cluster (yuck), but it's possible it's not the fault tolerance you're looking for.
It makes a huge difference in cost if some of the data needs to be continuously available, and it's mostly read-only. What I'm suggesting wouldn't address your read-write needs.