In the future it probably will be (if it isn't already). A new type of helper application (or "app") for the latest crop of cell phones (like the iPhone and the phones that use the Google Android platform) uses the phone's built-in camera to scan barcodes and then find better prices and product reviews. The most popular so far is called ShopSavvy. Here's a video of it in action:
A year ago, using cell phones as barcode scanners was on the horizon as something that might become commonplace (see NYT's "New Bar Codes Can Talk to Your Cellphone," 4/1/07). And here it is, now being used for shopping and boarding passes and all kinds of things!
The most literal library-type application would be a program that can connect a scanned barcode on a cell phone with results from WorldCat and forms to request it from the nearest library. Hopefully there are some library-loving app-developers reading this?
Google's Barcode Scanner app is sort of a step in that direction, hooking barcode scans into Google Book Search.
And some librarians are thinking in more social-web-like directions, like putting barcodes on a MySpace page to distribute program information, using barcode stickers to get people to the library website, or linking book exhibits to websites with supplemental info.
When every cell phone can scan barcodes, how will you use that capability in the library?