Apple today released iTunes 7.7, and within the past hour has launched their AppStore. iTunes 7.7 is required both for iPhone 2.0 software, as well as the AppStore. It can be downloaded from the iTunes 7.7 download page, as well as by using Apple Software Update.
In addition, Apple has placed on their download servers the iPhone 2.0 restore software. This will allow first-generation iPhone units to be upgraded to software version 2.0.
Installing the restore file will erase all data on the device. When fully released, iPhone 2.0 software will also offer an update option, which will fully backup the iPhone before updating. To update early, hold the Option (on Mac) or Shift (on Windows) keys when clicking the Restore button inside of iTunes, on the iPhone tab.