I just had a glimpse of a wonderful future where GitHub releases a payments system, packaging options, and suddenly becomes an ideal app store ecosystem.
With their recent $100mm round, they might be working on this already. Could lead to an interesting future of software selling/piracy, where those that will buy, buy, and those that are desperate not to can
Cyberduck, an FTP client, takes a similar (though easier) approach. You can buy it on the Mac App Store for $23.99 [0], get precompiled binaries for free on their website [1], or grab the source yourself [2].
I wish I could pay for Blink on Windows, because it's my main SIP client. Windows definitely doesn't get the attention that Mac OS does, but the client is the best one I've found. (I really grew to hate Bria/X-Lite.)
This can't really work on the long run / at scale, ya know... Someday someone will make a script that does the equivalent of the above, then somebody will make an alternative app-store GUI that leverages that script to get anything that's open-source for free. And you can't really sell as SaaS <$10 "apps" - so in the end you have two options: (a) classic closed source and (b) pay for subscription / user account (ok, there are the adds/in-app-purchases financed ones and they could profit from the "paid open source model").
Couple this with amerine's suggestion to allow release packages from private repos, and developers could also use GitHub to sell closed-source software.
I blogged a bit about this, with discovery and an app store type area with 3rd party apps like my site could be of use and extend features beyond what GitHub wants to support.
If you want to read about my ideas: http://derekmyers.com/posts/open-source-information-overload...