I am quite excited at the MySQL Labs feature to have a Binary log API .
The reason is that I see this as a possible disk-based queuing system for MySQL.
An example of how you could do this, is instead of having a trigger with its own overhead, you could create a program to parse the binary log and run a trigger yourself a little bit later with no "real-time" overhead. As in, it would be run when the user isn't waiting for it.
You could also, for another example, develop a program that counts the rows in a table and update memcached directly, instead of querying MySQL for it.
This can already be done with other queuing systems like Gearman, but with this API, you can develop other APIs or applications on top of it. Like, memcache_table_counter demon.
In any case, it would be very interesting to see how this will turn out. I believe it would largely depend on the community of developers who will use this API.