To install run this from the directory you unzip the file to:
adb remount adb shell rm /system/build.prop adb push froyo.prop /system/build.prop adb shell reboot
A message should pop up into the status bar after a few seconds depending on your connection speed saying checkin succeeded.
Now you're good to go, open Market and download away
EDIT: Since I know some of you will wonder what this does, it just changes the ro.build.fingerprint & ro.build.description settings back to ERE27. This alone would normally be enough, but you're depending on luck to when you next check in. Dialing that code in the Dialer will force a checkin.
EDIT 2: Thanks to Shafty023 here is a method for the shell:
LINUX SHELL METHOD
Copy froyo.zip to /sdcard, then open a Terminal shell on your phone
su mount -o remount,rw /system cd /system mv build.prop new.build.prop unzip /sdcard/froyo.zip mv froyo.prop build.prop rm /sdcard/froyo.zip reboot