Days of Wonder have released Ticket to Ride for iPad - (Ticket to Ride on the iTunes store) and it looks great.

It comes with the original game map and has the Europe, Switzerland and USA 1910 maps available as in-game purchases. It looks like they have a launch week promotion where you can get all the expansions for $0.99 each.  You can play solo against up to 4 AI players, or online via the iPad’s WiFi or 3G connection.