I'm also currently on a Treo (680, it's the one with the largest display, since I came from a Tungsten T3).
I don't use it as a phone per se, I still use it as a PDA. The phone part is just so I don't have to lug around a separate device. I very rarely need any Internet connectivity, I really like having everything local.
* Calendar: multiple categories (layers) of schedules, reminders, birthdays, vacations, ...
* Address Book: not just a phone book, but a full-fledged listing with addresses (home/work and postal/email), dates (birthdays, anniversaries), personal notes, ...
* Memos: quickly jotted notes-to-self, journal entries, inspiration, misc. reference, ...
* To-Do list: I'm a list-making nut, esp. nested ones (that is, outlines). Because of this (or therefore?), I'm a fan of David Allen's GTD method, and make heavy use of Natara Bonsai for my outlining needs.
* Entertainment: a couple of ebooks and games, no music.
* Reference: Dictionary&thesaurus, astronomy, world clock, chem table, ... (biorythms, too, but just for laughs)
* Security: Automated and encrypted nightly backups (in case of fatal crash), remote lock/wipe software (in case it gets lost), per-entry privacy settings in every app (I'm soo gonna miss that on Android, as have seen others lamenting for that functionality)
* Phone: occasional phone call, texting
Hey, anyone know of a good outlining text editor for Android? Or will I have to write my own?