You are reading an article from Wired or NYT. You like what you read and want to twitt. Alas, you are reading a paper version, not on-line. So, what do you do? You can either give up the idea of twitting or you can go on to your computer to find a digital copy of the article and twitt.
How about if publishers include little 2D bar code at the end of each article that act like "share" tag at the end of digital copy of the same articles? Readers can use the camera on their mobile phone to scan the 2D bar code -- depending on the mobile client the reader uses, it will allow you to share it via Twitter, Facebook, or many other social networking tools.
The same idea can be applied for other physical artifacts: books, restaurants, CDs, DVDs, movie & concert tickets, museums, mural arts, etc. Most of these physical artifacts have digital representation somewhere on the net. 2D bar code can simply provide URL for those digital representations so that it can be included in twitt (or whatever form of media the use may choose).