There are a few ways I've seen of displaying field-like inputs on the web without storing them in the submitted document, but I think what I'm about to describe is the easiest way of all.
Basically you don't give the field a name, but give it an ID instead. For example:
<input id="ToDo_tmp" type="text" />
In the browser this looks and acts like a real field. However, when you come to submit the form it isn't counted as a "Successful Control" and its name/value isn't sent to the server.
These kind of fields are really useful sometimes. As a simple example take the ToDo list form from the other week. As I mentioned then, the code added the given value to a set of hidden fields, all with the same name, which can be saved in a multi-value field.
Or, for you Prototype fans:
Obviously not a great example, but you get the idea, right? To add the temporary field you can use PassThru HTML and you can even give it a value using Computed Text. Note that this applies to all field types. Not just plain text ones.