Usability 
I've been pretty disappointed with attempts to offer online event ticketing via the mobile web. The sites I've looked at have been marred by pretty serious usability issues. A lot of the problems are related to payment. There really aren't any convenient ways to pay for purchases on the mobile web that are also widely adopted. The mobile versions of PayPal and Google Checkout are easy to use but don't seem to have caught on with consumers or merchants. On the full web most purchasing is done with credit or debit cards. For mobile, purchasing with plastic is too complicated to be very practical. You need to enter the 16 digit card number, full billing address, the card's expiration date and 3 0r 4 digit security code. Way too much typing. Barely acceptable with QWERTY, it's an intolerable pain with a standard 12 key mobile keypad. Mobile commerce desperately needs some sort of seamless payment system built into the phone or network.
I just ran across the US mobile website of Ticketmaster, the 800 pound gorilla of the online event ticketing business, at ticketmaster.mobi and was pleasantly surprised. I'm no fan of Ticketmaster's exorbitant service charges which can add 35% to the cost of already expensive concert tickets. On the other hand, I have to give Ticketmaster credit for solving most of the usability problems of mobile online ticketing.
Ticketmaster's mobile site is uninspiring visually but a model of simple effective design. Enter an artist or sports team or choose from a list of categories and you are prompted for a zipcode or city and state. You are then shown a paginated listing of matching events in chronological order starting with today. Clicking an event brings up a page with event and venue details and a click to call number to order tickets. The whole process can be accomplished quickly with a minimum number of clicks. Making a phone call may be low tech, but it's still the easiest way to complete a complex credit card transaction with a mobile phone.
The site's not prefect, however. Prices are not always listed and do not include those nasty fees. Ticketmaster conveniently remembers the last location you used if you entered a city and state but curiously not if you used a zipcode.