We have begun to use MySQL 5.1 and one of my colleagues, Yuan Wei, will describe the project at the MySQL User Conference. By we I mean my place of work, but I don't speak for them so this isn't a press release (although they do provide a great blog service).
The project maintains a persistent log of row changes. We modified MySQL to generate a log that is similar to the row-based replication log except the output is much easier to parse and is text-based. Another tool parses this log on a slave and appends a protocol buffer to a file in GFS for each entry in the log. This is guaranteed to not lose data. The file in GFS can be used by applications that want to be notified when particular rows have changed. It can also be used to maintain a copy of the MySQL database in Bigtable or the open-source equivalent. There is little delay from the time the row is modified on a master to the time the protocol buffer is written to a file in GFS.
You should go to Yuan's talk to get all of the details. The useful parts of this will be published.
This was based on MySQL 5.1 so it could reuse and extend support for log tables that is new in 5.1. I also want to start using 5.1 in some form to get experience with it -- I am interested in the InnoDB plugin and partitioning. And as much as I claim that I don't want new features, I want foreign keys. While that won't be in 5.1, it might be easier to backport it to 5.1 than to 5.0.