Damn... I run a test here to see if it was working in any page, and it wasn't!!!
The DatePicker doesn't work at all.
The funny thing is that before I use in the page, I tested and it was working...
EDIT: I tried EVERYTHING! Nothing seems to work... I even downloaded the "cupertino" theme again from the jquery-ui.
See, when I download the "cupertino" theme, I uncheck some options from the Widgets (Accordion, Autocomplete, Slider, Tabs, Progressbar) because I won't use them, and I really don't think this is the problem.