The funny thing is that Sublime Text is basically already available for free. I know a bunch of people who have been using the unregistered version for a long time, either because they are cheap or they just can’t afford to buy it (students).
How about not open-sourcing the program in the strict sense, but just releasing the source-code under a non-free license? For example, to use the source you must have a valid ST license, and you may not use the source in other programs, and may not redistribute it commercially? And you have to redistribute the source with the binaries if you make any changes (a bit like GPL)?
This would not expand the circle of those who are not paying for ST. People who don’t care about Jon’s income, or who can’t afford buying ST, already pirate it, and will continue. People who do care, and who do obey licenses, would still pay for it.
Such a non-free license would still let you customize ST for personal use. Someone could implement the long-requested feature of file-type icons, for example. If a new feature is well-coded, then it could even be ported back into the main branch.