To turn off iPhone auto correction feature (predictive dictionary) on textboxes, add autocorrect="off" attribute to your input tag.
<input type="text" id="keyword" name="keyword" autocorrect="off" />
You can also turn off auto capitalising/capitalizing feature by adding autocapitalize="off" attribute.
<input type="text" id="keyword" name="keyword" autocorrect="off" autocapitalize="off" />
And lastly, you can turn off auto complete in a similar way.
<input type="text" id="keyword" name="keyword" autocorrect="off" autocapitalize="off" autocomplete="off" />
More HTML tips for iPhone web apps can be found in Building iPhone Apps with HTML, CSS, and JavaScript