Oh yeah, I wrote proxy for MySQL in Perl (http://code.google.com/p/proximo/, http://xb95.dreamwidth.org/tag/proximo) which is actually a really damn nifty bit of software. It's just pretty slow and not that useful in production, if it was sped, it could be amazingly useful.

It can do the sort of fallback you're talking about. It can also force stupid applications that don't know how to split up reads and writes to different databases to actually take advantage of a slave. And some other neat things.

But anyway.

Yeah, MySQL replication doesn't give HA. If you want true HA, you can look at MySQL Cluster, which is a certain technology and supposedly works pretty well. I haven't played with that since 2004 or so, when it didn't work very well at all.