The spinner plugin was removed following 1.6.2rc2 along with autocomplete and the color plugin. For more information see (where it refers to 1.6.3 which, a few patches later, became 1.7)...
http://blog.jquery.com/2008/12/11/whats-up-with-jquery-ui/
http://blog.jquery.com/2009/02/12/jquery-ui-17-is-the-new-16/
The 1.6rc3 release is being wrapped up
and will be out within a week. Our
goal is to have 1.6 final out still in
December. We’ve decided to cut back
the number of new additions in order
to re-factor the existing plugins for
improved accessibility, performance,
and theming. In this release, the only
new plugin will be the determinate
progress bar. The autocomplete,
colorpicker, and spinner plugins have
been moved into a future release (to
allow time for further refactoring,
not delaying 1.6 any further), while
magnifier has been moved into
experimental again, soon to be
released as external plugin at Paul
Bakaus’ homepage (more on that in the
next final release announcement). With
a better planning process in place now
(see below), we’re confident removing
plugins from a release candidate won’t
happen in the future, and we apologize
for any trouble it might have
introduced for you.