I think having a donation system in place on the phone/tablet itself would be helpful and is much more likely to generate donations than simply a Paypal button on the program's website alone.

Instead of the user having to hunt down the official developer's website through Google (and potentially not getting there at all), then going through all the Paypal procedure... the user has to go through a lot of effort to donate that way (figuratively speaking). With a simple donate button in the application manager (or whatever gets implemented) which will automatically donate from the user's Paypal/Nokia Money/whatever account, more people are going to be willing to donate because the donation process is a lot less painful.

But as stated earlier, I'm not sure it would generate enough cash anyways to make it a worthwhile project of Nokia's.