If you are referring to micro-transactions whereby readers pay content authors, tipjoy and others have already tried the "e-tip" model and failed.

Also, paypal's donate feature has existed for some time now but it doesn't seem to be able to replace ads. If anybody could implement the safe, seamless microtransaction model I'd think it'd be paypal as they have the existing infrastructure already. Either they have overlooked it or they've decided that the model wouldn't work.