I think if you set it up your contacts and calandre will sync but im not sure about mail and notes
Mostly / Sort of...
Messages don't sync, mail settings do, but assuming you're using IMAP (with MobileMe or something like Gmail) then all the messages are stored on the server (with local copies on the device) anyway, so from the user's point of view as long as both the iMac and the Touch have got an internet connection, then both are up-to-date. I have an MBA and a Touch and use Mail on both. All folders are replicated across, all deletes, moves etc. It just works, ime.
Contacts and Calendars can sync either via mobileme (wirelessly) or via the ipod usb cable using iTunes. Both work well, though there's a one-time fiddle if using the itunes method to get your calendar colours to match if you have multiple calendars (see
http://www.touchtip.com/iphone-and-...calendar-color-fix-for-iphone-and-ipod-touch/ though in my experience it often requires quite a bit of trial and error). Once set up, again, it 'just works' ime. Updates to either device will update the other as soon as they are connected together, or as soon as mobileme syncs.
There is no to-do app on iPhone or iPod Touch, and this annoys the hell out of me. I cannot find any solution that does proper syncing with to-dos in ical. Apple just don't seem to consider this a worthwhile feature.
Notes, I don't know. I have notes on my iPod, and I have mobileme set to sync notes, but I don't think there's anywhere on the mac for them to sync to - I think it's just for syncing between multiple ipods/iphones, but I hope someone can tell me I'm wrong, and how to get at them on my Mac. What would be really nice is if there were dashboard stickies sync.
The main difference between mail on the 2 devices is that the ipod doesn't have a unified inbox if you have multiple mail accounts. So for example, on my MBA I can look at my individual work inbox, my mobileme inbox etc, but I can also look at them all at once by just clicking "Inbox". The iPod only has the individual inboxes. I don't find this much of an issue.