The reason this all came about is because I got asked to write some features for an existing GPL product. The existing devs are trying to persuade me that 'free' means 'as in speech' - it doesn't mean that I can't charge for the features once they're finished.

However, this would be a major undertaking (perhaps up to a year's work) and whilst the Free Software Foundation states that you may charge for your work, my interpretation is that you can (realistically) only charge for the first sale. After you've released that first copy, anyone can obtain the source, build your app and distribute it for free. This is perfectly legal under the GPL. So yes, you can get paid - but (realistically) only once. Any further payment you receive for your work is entirely down to goodwill.

For me, this kinda explains why Linux has got such a small market share. Over the years I've listened to countless Linux fans bleating about Microsoft's 'dirty tricks' and yet it's the GPL that must surely be shooting Linux in the foot.