I work for a music software company and we are thinking of developing a product that fretless and double bassist (as well as violinists, cello players, any fretless instrument) may find of use. It would basically be a "finger finder" application that tells you when you were fingering correctly and your intonation was on or off.
It would be used with a PC or MAC with a Mic or a using the audio in for the computer. It would also track progress (i.e. if you are practicing the C major scale, it would show you where you were hitting the notes and where they should be, giving you a score).

Do you think this would be a useful application? What other features might be helpful?

Could be useful as a practicing tool for sure. When you play together with instruments like guitar or piano you hear pretty well if you're on or off, but when you're practicing alone you slip out of tune easier.

The tuner would have to be very fast trigging to work properly. It would be helpful if it also could analyze recorded data. A mic is not that necessary; it's far easier to just plug the bass straight into the soundcard.

Any possibility it could also work for vocals?

i think it would be a better product if you skipped the computer element and maybe you could just plug into it and use headphones or something like that, like an actual device, not just software. something like the beatnik for drummers, check it out (http://www.musiciansfriend.com/product/Beatnik-Rhythmic-Analyzer?sku=449770)

Thanks for the feedback. We are working on another vocal product (that is more of a vocal method) that would also incorporate this same pitch recognition technology.

I wouldn't go for it. I can do the same with a tuner (mine comes with a little mic, so I can use it for acoustic instruments) and it's much better to play along with a recording that is intonated correctly, because it develops your ears, which is the end goal over knowing where to put your fingers. Fingers get fat (Yngwie...) and you can switch string gauges or instruments all together. But if you know what the note should sound like, you'll find the correct positions a lot faster.

I kinda like the idea being I've recently started playing DB. But working on intonation is two fold. First playing unaccompanied if you use a device too much you don't develop the sense of where notes are and the sound of intune notes. I use by rack mount tuner it is a good tuner, but with double bass it is slow to respone or sometimes read a 5th off before settling down. So I keep my tuner off the the side so I don't look at it all the time and force myself to focus on playing intune by ear.

Then the other part of the thing is playing inture with others. That could be simulated with a computer. Playalong exercises and tracks. Then some way to connect instrument to computers and be able to monitor and track intonation. Be nice to record the player with the track so they can hear back how well in tune they are. Again IMO the hard part for acoustic instrument is finding a pickup or mic with circuit that responds fast and accurate.