I have to ask: why?
Some of the articles talk about the benefits of QR codes, how they act as URL shorteners. I have to ask: who is retyping a link from an email newsletter into their browser's address bar? Email newsletters are like any other web delivered information, in that they can include links like this one that doesn't have to show the whole URL. Even if it did, it's still a clickable link.
From a user standpoint, which is easier: clicking a link (as you've been trained to do), or reach into your pants pocket, pull out your smartphone, fire it up, launch your QR code scanner, wait for it to scan the code, and be (finally) taken to the web page in question?
Now, that's not to say there's no place for QR codes in email marketing. You may be trying to engage customers who are smartphone users and get them more engaged. (Remember those Columbia Record one penny deals? Getting people to take the extra step of taping a penny Columbia House provided increased conversion rates enough to more than offset the cost of “lost” pennies.)
You might also be trying to cash in on the novelty of QR codes, and in so, be branding your business as ahead of the curve. That novelty will soon wear off, however.
I'm excited about the possibility of QR codes in marketing and communication, but I feel they're a tool that will work best in certain situations, for certain industries, and for certain audiences. This is a not a one-size-fits-all marketing solution, so don't get swept up in the hype.
[Update: I just posted a new article on QR code marketing here.]