Developer Mach Kobayashi has stepped up to fulfill this need with his new application Mach Dice [$0.99, App Store]. The $0.99 application provides a realistic dice simulation that can be trigged by button or by shaking your iPhone. Unique to Mach Dice is the ability to roll an arbitrary number of arbitrarily sided dice.
This video provides a good demo of the app and its capabilities:
App Store Link: MachDice