I have similar experiences, some kind people will donate, but (compared to the installed userbase) it's really just that - a donation, a little greenback thank you, not something you can base your N900 acquisition on.

There doesn't necessarily need to be a store, however (that kills the Open Source segment a bit and brings DRM into play). The best I have come up with so far was a very-cheap donation/subscription service (or maybe even a slight Nokia subvention) which lands money in a big melting pot. Now, as we know extras is centralized and we have stats about it, the money from the pot can be split across the projects proportionally to their popularity in that month. That way, more serious projects get more money, there is actually an incentive to make polished end-user software. and they have to make no license or DRM compromise. Sure, given the GPL you could mirror and install from other places, but considering the extra effort I don't think it will be a significant drain if the cost to the end user is low enough.

Pasted this to brainstorm. Oh, and if this goes through I expect a medal, a N900 and a job at Nokia, not necessarily in that order