To solicit a tip in bitcoin, one merely needs to provide a Bitcoin address for the sender to use. Because it is a direct transaction, there is no intermediary sucking fees from the transaction (except for the bitcoin transaction fees, which cost about penny at current levels).
Here is just a subset of those doing exactly this: - http://en.bitcoin.it/wiki/Donation-accepting_organizations_and_projects
However, if the recipient doesn't already know of Bitcoin or doesn't accept bitcoins currently, two services can be used to give an amount that is held in escrow by the intermediary and the recipient then can claim the funds using the intermediary's web site. This is something Flattr just recently added as well -- they refer to it as "unclaimed things": http://flattr.com/support/unclaimed
These two services that do this with bitcoins are: http://www.YouTipIt.org
YouTipIt lets you send a tip to any TipIt already registered on their site but also they offer the ability to send a tip to any Facebook user. If the user is not already registered with YouTipIt, the funds will be held in escrow until they are claimed by the Facebook user.
PayBitBack follows the same concept except theirs is a Twitter based escrow system.