rjray speaks wisely. The only thing I would add is that all your funky javascript code will be for nought, should the user happen to have javascript deactivated.

A second possibility is that someone will pick apart your form and figure out what values the server is looking for, and craft their own response, completely bypassing the validations the javascript code performs.

Never, ever trust anything coming from the client. Buried in a recent thread on web site design (or lack thereof) are some pointers to some very good sources of information that it would behoove you to read.