There's a recent study out from Convio that's worth a look: The Next Generation of American Giving (PDF; registration required).
Basically, it tells us Pay attention to online fundraising. Which is absolutely sound and correct. If you aren't doing at least some of your fundraising online, you are setting yourself up for a world of hurt within a few short years.
But the study has two very serious flaws that limit its real-life usefulness:
1. It relies on self-reported data.
Anyone who has compared what donors tell us in surveys and focus groups to actual response data is familiar with the gap between what they say and what they do. Self-reported data can give you hints at what's going on with people, but if you want to know the truth -- as opposed to people's wild guesses -- go with solid response data. Nothing else will do.
2. It spends a lot of time looking at the giving behavior of young people.
This data can send you down some very scary garden paths. Looking at people in their 20s and 30s is almost entirely a waste of time that reveals pointless and confusing information. Because young people are not the audience for fundraising. Their habits and opinions pretty much meaningless in our world. Studying young people is like studying newts to understand the behavior of crocodiles; there might be some connections that yield insights, but it's not the richest area of study.
I still recommend you read the study, there's good stuff in it. Just take you read with a grain of salt. I'm with The Agitator's comment on the study:
Hopefully most direct response fundraisers will follow the reality presented by their own hard data. For virtually all nonprofits, their actual giving data will continue to justify strong investment in direct mail programs ... even as they begin to see their online giving ramp up.