Two comments on the circuit:

1. If you're willing to add two more diodes, you can make the USB connector two-sided so that it can plug in either way. See http://electronics.stackexchange.com/questions/209941/two-si... for explanation.

2. The ALPS SKQGAKE010 (http://www.mouser.com/search/ProductDetail.aspx?R=0virtualke...) is inexpensive and popular. It looks like it's lower-profile than the button used in the current design, which means it'd be more likely to survive for a long time in a pants pocket, jangling along with a bunch of keys.