But if you are running a rooted Nexus One, you'll need to follow another guide to get Froyo running. You'll also come to realize that while paid apps are visible once your carrier ID has been faked (see Market Enabler), encrypted or protected apps are not visible. Documents To Go, the New York Times Reader, ExZeus are a few I noticed.
To get these back, you'll need to do the following:
1. Connect your phone via adb and enable USB debugging
2. Download 'froyo.zip' from here and extract its content (froyo.prop)
3. Type the following
adb shell cp /system/froyo.prop /system/build.prop.bak
adb push froyo.prop /system/build.prop
adb shell reboot
4. Once reboot has been completed, checkin by entering the following in your dialer
5. You should see a notification in the status bar.
And you're done :)
This is just a temporary fix. Once Froyo is released officially, AndroidMarket will recognize it as a valid device and allow unrestricted access to protected apps. When this happens, simple delete 'build.props' and rename 'build.props.bak' to 'build.props'.