Sometimes the spell checker is unnecessary, captchas, usernames, emails, etc. Is there a way disable it on some text inputs/text areas?
As noted by my buddy Jonathan Stark, you should be able to use attributes like autocorrect and autocomplete to achieve the effect you're looking for:
<input type="text" placeholder="My Field" name="myfield"
autocapitalize="none" autocorrect="off" autocomplete="off" />
Just tested in OS4 and it's working.
UPDATE: As of iOS5 on and off values in autocapitalize have been deprecated. Use none to completely disable automatic capitalization.