[robgough]

If they were working from home, sure :P

I can't see it impacting on their work, but it might irritate others.

There is an obvious second question though, how would you know before you employed them. The question then becomes, would you fire an developer that talks to themselves at work.

In which case, No - I probably wouldn't. At least not without trying to find other ways of dealing with it, it is possible they don't even know they're doing it - and after making them aware you might find that they stop.