Yesterday I tweeted that you may not need to reboot in order to upgrade to Lion. That isn't exactly true. After watching yesterday's Keynote from Apple I realized where the Engadget editor made the mistake.
Typically when you insert a disc to upgrade Mac OS you need to reboot to the disc for the installation process to begin. This means that during the entire upgrade process (even though this process gets shorter and shorter with each update) you'd be effectively without a computer. The upgrade to Lion, it would seem, doesn't require this step. The update happens just like any other installation process. However, you'll still need to reboot when the upgrade process is complete.
Notable, but not as notable as not having to reboot at all.