Just checked out CellFire, which lets you store coupons on you cell phone (then, allegedly, the checkout person will enter the 'code').
It reminded me of the application needed for the G1 (no, not another flashlight or tip calculator). There's already a barcode reader (one of the standard 'wow' apps on the G1), which provides barcode services to the two main price comparison apps. What we need is a barcode generator. So I can scan all the store 'savings' cards I don't carry around, and be able to reproduce them when needed. A virtual version of justoneclubcard.com.
Of course, I'm not sure how well scanning the generated barcodes would work (although I seem to remember reading about some guy who had success), but just displaying the barcode and the associated data would allow manually entering in the 'savings' card id.
But until the savings card wallet app comes around, I guess there are other interesting things to do with barcodes.