I've often thought similarly and was going to post a similar problem. Sometimes I'll read a post and hit (++) then realise I don't really want to vote it up after all. I'm left only with the option of reloading the page to remove my vote.

It would surely be a simple patch to just add a third radio button that just returned a zero (or, maybe a null string: some browsers might not bother passing the vote's name if it has a null value ... anyone know?)

<input type="radio" name="vote__XXXXXX" value="1" />++
<input type="radio" name="vote__XXXXXX" value="1" />0
<input type="radio" name="vote__XXXXXX" value="-1" />--
[download]

or looking better:

-- <input type="radio" name="vote__XXXXXX" value="-1" />
<input type="radio" name="vote__XXXXXX" value="0" checked />
<input type="radio" name="vote__XXXXXX" value="+1" /> ++
[download]

Which would look like:
-- ( ) (•) ( ) ++