I've written numerous times about 2D barcodes and use of camera phones as a search or advertising vehicle. In Japan the use of QR codes in traditional media and outdoor advertising is well established. Now in the US (as well as Europe) there's some movement toward adoption. A number of competing companies are promoting barcodes. 

Today, Rita Chang in AdAge writes about a successful use of mobile barcodes by Mexican fast food restaurant Qdoba:

[Qdoba] bought ads in the [University of Michigan] campus newspaper and posted promotional posters, each with a code kids could scan with their phones to get a mobile coupon for a buy-one-get-one-free burrito. The campaign netted a 52% redemption rate with about 400 scans, roughly 1% of the total target student population

This amazing 52% redemption rate is probably a response to the combined effect of:

• The offer itself
• The targeted population
• The novelty of the methodology 

The advertiser also gets to see, relatively speaking, which traditional media or marketing approach is more effective as a secondary benefit. Indeed, mobile barcodes (or their equivalent) make lots of sense across the board:

• The can work on feature phones as a way to get information without keying anything in
• As mentioned, they can tie in to traditional media providing response measurement and an immediate rich experience for consumers (see, for example, this ScanLife video). 
• They close the loop between the ad source and the point of sale

The challenge with most barcode systems, as the article points out, is that an application must first be downloaded. However in the iPhone apps store and potentially other smartphone apps stores, this becomes easier. On some phones the software may eventually come pre-loaded. An open-source standard would accelerate this process overall.

Codes in SMS messages can also do something equivalent in a simplified way, as HipCricket has been doing for traditional publishers and media outlets (e.g., radio and TV). One could even imagine a "bifurcated" system in which QR codes were available for those that had the capability to read them and a short code as an alternative, parallel way to get the same information/offer for those without it. 

Regardless, some version of this technology will probably take hold as one form of mobile marketing in the US and EU markets. It also makes sense for traditional publishers to adopt shortcodes or barcodes in their publications as a way to extend their value and make them more "measurable" in this time of declining confidence in traditional media.