Is there a W3C valid way to disable autocomplete in a HTML form?
Here is a good article from the MDC which explains the problems (and solutions) to form autocompletion. Microsoft has published something similar here, as well.
To be honest, if this is something important to your users, 'breaking' standards in this way seems appropriate. For example, Amazon uses the 'autocomplete' attribute quite a bit, and it seems to work well.
If you want to remove the warning entirely, you can use JavaScript to apply the attribute to browsers that support it (IE and Firefox are the important browsers) using someForm.setAttribute( "autocomplete", "off" ); someFormElm.setAttribute( "autocomplete", "off" );
Finally, if your site is using HTTPS, IE automatically turns off autocompletion (as do some other browsers, as far as I know).
Update
As this answer still gets quite a few upvotes, I just wanted to point out that in HTML5, you can use the 'autocomplete' attribute on your form element. See the documentation on W3C for it.
I would be very surprised if W3C would have proposed a way that would work with (X)HTML4. The autocomplete feature is entirely browser-based, and was introduced during the last years (well after the HTML4 standard was written).
Wouldn't be surprised if HTML5 would have one, though.
Edit: As I thought, HTML5 does have that feature. To define your page as HTML5, use the following doctype (i.e: put this as the very first text in your source code). Note that not all browsers support this standard, as it's still in draft-form.
<!DOCTYPE html>