Anyone can "dual boot"(boot without any effect to the already installed Maemo 5) MeeGo right now on your N900 if you want, but it isn't in a condition where end users would expect their OS to be.. hence "for developers".

To dual boot MeeGo now, you have to connect the phone to a computer with a USB cable and temporarily load the kernel when you want to boot MeeGo. "Easily dual boot" will remove the need for this.

Some people might've had the idea that when your PR1.3 OTA update is installed, you're greeted with a fancy menu asking which OS to boot. No, MeeGo isn't included in PR1.3. Anyone(developer or not) can get an image and kernel from meego.com and try it, but it's not something the PR1.3 update does for you. It doesn't happen automatically, or with just a couple of clicks as "consumers" might expect.