Get a list of names of attendees (and email addresses) in advance
Limit the numbers if you don't have a reasonable ratio of mentors to attendees (suggested max 1:4)
Mail attendees the day before to remind them to bring laptops (and possibly power supply and ethernet cable)
Ask attendees (with any other
OS than Windows) to try and download and compile
PHP in advance (to check that they have a build env't)
Make sure that the location has network connection (wireless/ethernet)
Pick an area (extension) to test in advance and let the attendees know so that they can do some preparation.
Mail the QA list to say what extension you are looking at so that you can co-ordinate with other groups if they want to work in the same area
Work out in advance which test cases are missing and have a list of then ready for people to work on
If you pick something that has pre-reqs (eg database, libraries) prepare some samples and make sure that people know to install the pre-reqs.
Either at the event or in advance have someone give a short talk on the extension and show example of how to use it
Be prepared for network failure - have copies of
PHP source, Windows builds and docs on a stick.
Find a local source of beer for *after* the event
This is all we thought of for the London one, I expect I'll add to it after the London event with all the things we forgot 
