Combine tipping with aggregators
With content we don't know how much it's worth until we've already consumed it (a classic measurement problem). A tip would solve this problem, but a tip is a social thing and requires more than just a payment.
If I were doing TipIt I'd change the design in four ways. First, I'd add generosity signal features that inform one's friends or fellow tippers as well as the tipee about the tip. This could be in the form of aggregator "karma" points that name and ranks the most generous tippers. This would be like the "karma" points which people who add content to a social aggregator compete for, but it signals far more -- it signals that one is a generous tipper as well as a generous contributor of reccomendations. There are a variety of other ways (home or facebook pages, e-mail, etc.) that generosity signals might similarly be sent within a social circle.
Secondly, I'd add it to an existing aggregator rather than starting a whole new service from scratch.
Third, I'd add some extra security (cryptography, natch :-)
Fourth, I wouldn't try to aggregate "microtips." I think these will have the same kinds of problems that micropayments have. Instead, I'd make the tips nice round numbers in increments of 50 cents or a dollar. That makes it simple for people to keep track of their own tips and their friend's tips.
So here's how it might work:
Your client software (perhaps just a web page script) pays the tip (using an already existing system like PayPal) to the content provider, perhaps via the aggregator, and publishes a cryptographic proof that you made the tip. The aggregator verifies the proof (if it's not the payment intermediary it does this by cross-checking with the tipee), registers the vote, and shows that you made the tip. This signals to others both that the content is appreciated, that you are a generous tipper (thus transporting the social incentive to tip from dining out to the Internet), that you have good taste, and that you put your money where your digg vote is.
Optionally, it could publish a bit of HTML code on the tipper's own blog or home page, that links to the aggregator and shows how much a person has tipped. It could send out e-mails to the tipper's friends recommending the content and, along the way, informing the friends of the generosity of the tip.
If content tipping on the Internet will work at all -- and given the measurement problem of content it would be very nice if it would work - I think it's much more likely to work well if done in a social fashion like this.