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...