For apps distributed through the App Store, Apple will deliver the crash logs to you through iTunes Connect.
For Ad-Hoc builds, you will need to have your users send you the crash logs. When the iPhone is synchronized with iTunes, it copies all the crash logs onto the computer. Here are the locations where they are stored on various OSes:
Mac OS X: ~/Library/Logs/CrashReporter/MobileDevice/[DEVICE_NAME]
Windows XP: C:\Documents and Settings\[USERNAME]\Application Data\Apple computer\Logs\CrashReporter\[DEVICE_NAME]
Windows Vista: C:\Users\[USERNAME]\AppData\Roaming\Apple computer\Logs\CrashReporter\MobileDevice\[DEVICE_NAME]
(I got these paths from Google; haven't verified them myself. But someone can comment if they're wrong).