Had a thought a few minutes ago about a more standards-compliant way of doing this.
You could generate a random string using a server-side language of your choice and set that as the name attribute of the input field for which you don't want autocomplete to detect.
You'll also need to set the same string as the value attribute of a hidden field which would also be passed to your form handling script.
The script could then grab the value of the hidden field to get the name of the text field. Does this make sense?