Because of the iPhone’s amazingly convenient mechanism for syncing with OS X’s Sync Services, I’m impatiently waiting for the non-Apple apps that I use to adopt sync services. For example, Daylite currently offers sync with the address book service, and the iPhone makes it surprisingly not painful to manage hundreds of contacts. The Verizon UI on my old LG phone, by contrast, was painful for managing just 20 contacts. Daylite is still working on calendar syncing which is going to be hugely useful for us.
On the email side of things, I use Thunderbird because I prefer the way Enigmail handles GPG over the extension for Mail.app, so email sync between the desktop and iPhone is currently a no-op, though IMAP is still an option (and probably a better one except perhaps for speed over a mobile network). On the browser side of things, getting Firefox’s bookmarks synced with Safari’s bookmarks collection would be highly useful because typing in urls on the iPhone is a lot more painful than using a traditional keyboard… though still less painful than with a normal cell phone or smart phone!