Just a few hints on how to get rolling.
1. Get a barcode scanner off of the market to read them with. Barcode Scanner
2. Create your QR Codes at Delivr.com Medium size works best.
3.Upload the attachment and post you semi secret message.
4.If you are using your phone(lol). Download the image and upload it here if you need it decoded online.
5.Found this on the market. QR Code Generator for the phone. Decodes and creates. Testing it out soon. Also QR Droid Private (Permissions changed slightly same app though).