Unfortunately, I'm not sure what to do about the weird behavior of the Fn key. I have a feeling it's just a problem with GTK spin controls, but I'm not really sure. Can some python/glade guru help out?

Anyway, I updated the first post with a new version -- version 1.1. It not only fixes the problem with changing the number of people (I had the control set to uneditable.. woops!), it also adds support for tax. Tax works just as it should: you can choose either percentage (if you know it in your area), or the price of tax (which most bills tell you). Enter the subtotal in the first box, then enter the tip and tax. Tip will be calculated off the total without tax, then tax will be added, and the final bill with tax will be split.