Propose a simple system for the user.
For the user:
When the user wishes to donate to a Free Software (FOSS), they can simply click on "Donate" in the software concerned. The "Manager of donation" of Maemo start and reminder the name of the software concerned (to avoid errors). The user specifies the amount, his user name and password (probably that of Maemo.org) and click to "Donate". It's simple ^_^.
The user can save the user name and, in addition to donations, he can write a little message of thanks to the developer.
The money has come from a virtual wallet, managed by Nokia or a non-profit association (to avoid abuse), the user can recharge in several way (credit-card, paypal, bank transfer, payment unique to his country (exemple: postfinance for Switzerland), etc ...). As simply and quickly as possible.
Donate which will return karma points, like any other contribution.
For the developer:
To use this service for his Free Software, the developer must register with those who manage the service (Nokia for exemple) and follow the instructions to allow his software to start "Manager of donation" for Maemo with good informations.
In addition, the developer can send a message of thanks to the donor.
For Nokia:
To promote this system and to accustom users to the principle of donation, Nokia will offer a prepaid card for about 20$ with each MID (like the N900).
May also, maybe, promote the message: "No pay for obtain, but pay for support the software and thus for it continued".