They've been doing that for a while, it encourages them to create bundles from other people's games rather than just their own. It's good press for the rest of the indie community!

I think the humble tip has been part of the humble bundle ever since the beginning. Not sure if they always named it that way though.

It's not been there since the beginning, I believe they added it in on the second or third bundle.

Pretty sure it was there on the first one, I remember setting 50/50 split between devs and Humble tip on my first purchase.

Nope, you're remembering wrong. From the announcement of the second bundle [1]:

However, new this time is a customizable "Humble Tip" which goes to fund a new company Jeff and myself recently started: Humble Bundle, Inc.

[1] http://blog.wolfire.com/2010/12/Humble-Indie-Bundle-2---IT-S...