Pocket informant ID the best I have found to replace datebook. I loved that program! PI is mature on the windows mobile platform but is still developing on the iPhone and should get better with the next update that was due in July.
Due to Apple's refusal to allow developer's anywhere near the calendar, there is nothing that comes close to DBook or Agendus on the iPhone. PI has a good start, and Agendus is getting ready to roll out calendar support to sync with Google. But until Apple lets go a little, we won't see anything close to what we enjoyed on the Palm.