There is a significant difference between what Martin and Peter did and the use of this system for new software projects.
GIMP Print and Firefox were both ports of existing software. People knew what they would get (more or less) before they pledged money. Of course, they didn't know how good the finished item would be, but they did have something to look at before work started.
With a new peice of software it might be rather more difficult as people would (presumably) be asked to pledge money based on a "concept" and "feature list".
Having said that, it could be very useful for projects like David Buck's new program which have reached the stage where a "proof of concept" already exists and where a pledge system would help him to decide how much serious interest there is and hence how much time and effort he should put into it.