I'm not sure if it's that simple. I think its a question of expectations. I believe that most people don't expect a phone number or URL to work underground because that's what they've come to expect over time. Chances are, at some point, if they've seen a number/URL underground and, assuming that they've felt the urge to follow it up immediately, they've failed and "learnt" that i's a no no ("Of course. I'm underground. No signal"). At present QR codes are to new for many people for the learning process to have taken place so they get negative experience when they scan off-grid. I believe that people expect something to happen when then scan a code. A "Can't connect" doesn't cut the mustard.
However, in many ways simply putting a URL in a code in such a situation is missing a trick. Much better to put a call to action around the code to manage expectations "Scan this code to link up later" and have a further call to action in the code content ("Remember to go to http://(shortURL) when you can)" or even an iCal event/todo (see http://code.google.com/p/zxing/wiki/BarcodeContents).
It's a good idea to have a code (faster and more accurate than scribbling down a number/URL) as long as it is location adapted.