I don't know if this is already recognized, but the most plugins which used the buttonsnap.php class from Owen Winkler didn't work anymore with the new TinyMCE version in 2.1 . This will bring a bigger problem for the most users, when a couple of plugins don't show the plugin button in the RTE.
I know that buttonsnap is not a core part of wordpress, but you should remind that a plenty of plugins have the requirement to show a button, and they use many times buttonsnap as the easiest solution.
At the moment it needs 6(!) action hooks/filter to show a simple button and some time to study the TinyMCE "plugin" documentation at moxiecode. Maybe there is some way to implement a core hook in the tinyMCE plugin folder , which assist a button integration in the way of the buttonsnap class.
On the other hand a extended codex description could be also very usefull...
regards
Alex