This started out as a set of internal guidelines for the staff at MPOW, but some friends expressed interest in it, and it seems to have struck a nerve, so I'm posting it here, so it is easier for people to find and to link to.

Using QR Codes in the Library

QR codes are new to North American, but have been around for a while in Japan, where they originated, and where everybody has a cellphone that can read the codes. They make it simpler to take information from the real world and load it into your phone. As such, they should only be used when the information will be useful for somebody on the go, and shouldn't normally be used if the person accessing the information will probably be on a computer to begin with.

Do Use QR Codes:

On posters and display projectors to guide users to mobile-friendly websites.
To share your contact information on posters, display projectors, or your business card. This makes it simpler for users to add you to their addressbook without having to type it all in.
In display cabinets or art exhibits to link to supplementary information about the items on display.

Don't use QR Codes:

to record your contact information in your email signature. Somebody reading your email can easily copy the information from your signature to their addressbook.
to share URLs for rich, or full-sized, websites. The only URLs you should be sharing via QR codes for are mobile-friendly sites.