Yes, it's a good idea and I like it too. It's OSS but you may pay for the convenience of getting the app perfectly packaged for your OS.

Other OSS projects use that model (I can think of Todo.txt Android app, for example).

But I'm not sure that it works exactly like "can you build it? It's free if you don't want to pay for it". OSS is about distribution. Anybody can build your app and distribute it legally (in this case Hacky is LGPL, it wouldn't be too difficult to comply with the license), even do it for a fee.

It's more paying for "the extras" straight to the developers. Call it support, iTunes, packaging, or just making things easier for people that don't want or don't have the time to get the hands dirty.

I like the model too and you got me thinking. Is there any standard licence model where you allow people to have and use the source but not to redistribute it "as is"?

That sounds like what Microsoft did (and got slammed for) with their Shared Source license.

The one thing with doing that myself that would worry me, is some community project releasing free pre-built versions would spring up.

That's essentially what happened when XChat (IRC program) started to charge for windows builds.

I'm aware of that example, it was on my mind when I made the comment.

I suppose the only way it could work is if there is a special incentive to buy it. Some video game companies do this, they have the engine open-source, but all the assets are proprietary.