Platform
To create a code: IE 9+, Firefox 3+, Chrome 3+, Safari 3+
To view a code: iPhone with Barcode Scanners that opens in Safari. Other phones/methods coming soon. Android does not support opening .ics files.
Steps
Task 1: Make a code
Go to http://rscanvp.com and click "Get Started". try filling out the form (it does not save your data). Try creating a code. (It does not save your settings as you more back and forward yet). Download the code (not implemented). Inset the code into your favorite graphic design program (ie Illustrator, Publisher).
Task 2: Scan a code
Scan the QR code in the following poster. The URL goes to http://rscanvp.com/view.php The sample ICS file is open-able in iPhone Safari (but not a web view in a different app), as well as on Windows with Outlook installed. (The Add to Google Calendar and set up email reminders is not implemented) 
Task 3: View RSVPs
Go to http://rscanvp.com/admin.php. See (fake) RSVP counts. Can edit event details and code (fake data is filled in)
To Be Completed
The creation of QR codes does not generate actual QR codes, but random binary checkerboards.
Downloading your QR code does not provide a downloaded version of a QR code.
The admin page that reveals the results of a user's QR code does not actually portray specific results, but a static image of possible results.
There is no way to email the list of attendees, instead a view of a fake list of them.
The data for a QR code that is input is not saved between pages.