Hi David,
I wanted to point out one team of software developers thats doing this right now, and actually has been for some time.
Thomas and Amy are framework developers that publish some of the most intense and widely used front end javascript frameworks on the web. As I'm sure you know every framework is released under the most liberal license that can be found, usually MIT or BSD and it would appear that its next to impossible to sell a copy of any framework these days. They seem to have found a really ingenious way to make a bundle selling ebooks! if you go to link here or link here you can see their ebook prominently displayed at the bottom.
I think this is a great way to profit from your endeavors without forcing your customers to pay up throat under boot. As a bonus, the level of notoriety they have reached in the framework community has brought them to speak at numerous conferences as well as co-author books.
I'm currently developing software that works on an alternative business model myself. It's based on affiliate offers, where in a user of my software is asked to use one of my affiliate links next time he/she buys something. We then unlock some minor feature that could also be had for free if they wanted to download the source from SVN and compile it.
--Richard Corsale