There is buzz about how Jason Kottke, of kottke.org, has abandoned his experiment of micropayment donations to support his full-time blogging. He pulled in $40,000 in the year, almost all of it during his 3 week pledge drive, but that's hardly enough. Now I think he should try adsense, but I doubt he hasn't heard that suggestion before.
However, PBS/NPR are able to get a large part of their budgets through pledge drives, so it's possible to make this happen. I think we should be able to do it better on the web.
For example, on PBS/NPR, when they start the pledge drive, they get into a pretty boring endless repeat of the basic message. They tell you that if they reach the goal, they can end the pledge drive early. But this rarely happens, and even when it does, if you pledge early, it doesn't stop the begging.
On the web it could. You could do a pledge drive here where, after a person donates, the drive is over for them. This is not the same as sites that simply charge a subscription fee to get past the ads (such as Salon and Slashdot). This would be an organized pledge drive which is over for everybody after a set period, but over even sooner for those who donate. (There's a touch of work to do for people who use multiple machines, of course.)
Indeed you could even have a "turn off pledge drive I'm never going to give" button for the freeloaders as an experiment. Or it might turn it down a notch. Hard to say if this would work. Of course, people could also write filters for web begging if you make the drives too long. Of course, the drive could even be started at an individual time for the less frequent visitors, though that punishes those who disable cookies or switch machines.