As Jabber does not seem appropriate for a SMS-replacement (the clients just don't to survive connection changes from wifi->3g, 3g->E, etc.) i've been looking for something different. E-Mail is nice, just the shortest polling-interval of 15minutes is not suited for realtime communication. So - theres Push Email (meaning the iPhone will hold a HTTP connection to some server open all the time and the server writes something to that stream as soon as a new mail arrives) for various services including Microsoft Exchange and GMail. How about you hosting a own server since you don't want to give away all your data? No problem - either use one of the tons of commercial servers supporting Push or use Z-Push with your favorite IMAP and SMTP server.
I recently got me a iPhone for tinkering, development (i've got a few nice ideas) and general nerdism. I've run into a few problems syncing my PIM (stuff like Contacts, Tasks, etc.) - especially since i use GNU/Linux which is no platform to run iTunes. Pictures and Music is no Problem as gtkpod and the like support the iPhone. Just the important stuff does not work out nicely.