error with ui tabs

Hey folks, I am having a major problem with jquery ui tabs here....I
do not understand for the life of me why this is happening because
i've used it just fine in many other sites before. To try and debug a
little I switched to the non-minified version; the error in safari and
firefox's error console is:
TypeError: Result of expression '$[namespace][name]' [undefined] is
not a constructor.
(line 246 in jquery-ui.js)
It only happens when i have $("#tabs").tabs(); in the document ready
function; if I comment out the tabs then the error disappears. But of
course, then I don't have tabs.