Given that a fretless has the ability to scoop into notes, and to slide, AutoTune set on the Auto mode would sound just as hard as on the Cher hit "Believe," only no one would notice that a fretless was being used.
I suppose that one could use AutoTune in the Graphical mode, which allows one to decide how extensive the correction will be in any given section.
Here's a page which gives some great examples of what AutoTune sounds like and how it works. You can extrapolate from the voice examples and imagine what would happen with a fretless.
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Having known guys who would actually edit their MIDI data to the point of losing all spontaneity, I don't know how useful it would be to spend all that time on editing a single performance every time, as opposed to spending that time woodshedding to bring a fretless... or one's technique... under control.
Anyway, if it's just to hear if it's a cool effect, I imagine it would be invisible to a listener, as most basses are fretted and are constrained to those pitches, unless one does any kind of bends or vibrato. In that case, you lose all of that.
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Being a little less serious, the following pedal, although expensive, goes a little further in improving things if one's goal is to improve a bassline.
From the website:
"A pedal of the greatest magnitude!! I've seen all the grousing on the various discussion boards about "busy" bass players in blues bands and the grief they can cause. Give 'em a break guys and girls, it's not their fault! It's the dreaded Jaco Syndrome (no disrespect intended!). Simply insert the Jaco-B-Gone in your bass players signal chain and he or she will never know what hit them! No matter how hard they try, all that comes out is a deep, grooving, solid bass line."
