MySQL has announced two very useful new features for versions 5.1.5 and 5.1.6.
5.1.5 will get new XML capabilities and 5.1.6 will be able to schedule event tasks. That will be very helpful to clean up outdated data, for example - a task that had to be done on the client side so far, but now it can be delegated to the database server.
I still wonder if MySQL 5.1 will introduce check constraints, as this is still another thing that still has to be done on the client side, although it would be useful to transfer this to the database server.
Another thing that should abolutely be in 5.1 are separate privileges for trigger creation and deletion.
If these both features, together with the features that have already been announced, will make it into 5.1, I'm sure that MySQL 5.1 will become a very important new release that brings it a big further step forward to compete with the biggest commercial database systems.