[ekianjo]

Make them pay in advance and reimburse the fees if they show up. Problem fixed.

[yellow_lead]

This creates a new bias against people who can't lend a conference $X,XXX for a flight and hotel.

[threatofrain]

Conferences are generally so expensive that early bird vs not is no longer the question at the forefront, but rather the sheer largeness of the number. Also early birds tend to save money on flights and conferences.