I don't know if there exists good tutorials (though I have a good book on
the subject)
but the best way to learn how everything works is to look at the source code
of the look & feel classes within the JDK source code you can find on Sun's
website.
Nevertheless, if you look at this code you will find that the best way to
have round
corners is to create a RoundedBorder ;-) (i.e. you won't need to modify the
Button UI) In fact, most of the time you won't need to modify this class to
achieve what you want, the only reason to modify it would be to change its
behaviour.
Last point you should also know that the Java L&F classes are sometimes
a little bit "buggy" when deadling with special L&F due to lazys Sun's
programmers. I have pointed this problem with the Sun's Java bug parade
but have not checked yet if changes have been included within Tiger.

Hope this helps

Cedric Lefebvre
cedric.lefebvre@nospam.free.fr