Hello!!! Froyo's not even officially out for the Nexus, what do you expect? Be patient, sooner or later they will make it available. Of course, it should be backwards compatible, but since the droid is using a ported version of the pre-release Froyo, I'm not surprized.
just did the froyo update w a custom rom, rooted, added busybox, then 900mhz core, then market fix all on my moto a855. paid apps all showed back up all work but skype mobile and it runs circles around what it used to do. love froyo