See the screenshots in this blog entry:
http://weblogs.java.net/blog/campbell/archive/2007/02/swing_and_gtk_w_1.html

Note that tabs (at least in the TOP and BOTTOM placements) are still 4 pixels taller
in Swing than in a comparable native app. This issue was described in detail in
6354790 (including a partial fix), but the issue wasn't resolved with the fix
for 6354790.
Also note that the gap between "Tab 2" and "Tab 3" in those screenshots looks
different than in native. That issue needs to be addressed as well.