I have a simple form with an external action that I want to validate with the jquery.validate plugin. I cobbled things together in a flat html file, including links to the Microsoft CDN for the relevant scripts, and got everything styled up and looking decent. I wanted inline validation, so that when someone entered valid input then tabbed out, they would get visible feedback, like the demo here:
http://docs.jquery.com/Plugins/Validation
I cut and pasted the form into an unfiltered node and added a link to the jquery.validate plugin to the footer of html.tpl.php. Nothing happened. I got the classic "$ is not defined" error in firebug.
I built the original form with jQuery 1.5.2, rather than d7's 1.4.x, so I installed the jQuery Update module, activated it, and selected the Microsoft CDN option in the performance configuration. I went back to the form, and again, nothing happened.
To check for user error, I edited html.tpl.php again, this time adding an external link to jQuery 1.5.2 after the print $scripts; code. The form validation worked -- everything was as desired.