Ovi has an infrastructure in place already. You have registered developers, registered users, a catalog of apps and a payment framework. You have also a delivery channel for free applications. Getting a first implementation allowing voluntary payments for free apps doesn't look like a huge amount of extra work.

I guess it's clear to everyone that a donation system is not a substitute of your salary, and not even a full return of investment. If you really care about monetary income for your software then go commercial.

Also note that Mozilla, Ubuntu, GNOME, KDE, Wikipedia, CreativeCommons and etc have all Donate buttons linked to PayPal and the likes...

... but now it's me not putting ideas in the Brainstorm! I want to shake things more than pushing them directly. At the end I'm not a developer myself.