Nowadays, many airlines, movie theaters, museums and other venues allow you to buy your tickets online. It can be a great time saver and convenience. All you generally need is your credit card and a printer.
However, as a consumer and a marketer, it really appalls me that I have to waste paper to show that I purchased something, especially since there is at least one green option: mobile phones.
Given mobile phone's current penetration - nearing 61% worldwide, according to a recent ClickZ article - it's very likely your customers are carrying a mobile device.
Thus, I would encourage all companies that offer "buy online options" to give three choices for ticket purchase/admission entry 1) allow to the purchaser to receive a confirmation/ticket via text message 2) allow the purchaser to receive an email formatted for a mobile phone/PDA for display upon entry 3) allow the purchaser to print the actual ticket via an e-mail sent to their computer (for those that don't like the mobile device option).
These options may take a little IT support to implement. But customers will appreciate the added convenience, especially when local search becomes commonplace and a printer is nowhere in sight.
In the interim, I strongly believe that greening the e-ticket process is a good idea. Not only can it spare trees, but it could enhance customers' opinion of the company, and potentially tie them closer to the brand.