When iTunes first appeared (January 9th, 2001, fact fans), it pioneered the brushed metal effect that has delighted and annoyed Mac users ever since. In interface design terms, it was a trend setter. Brushed metal slowly infected other apps, even the OS itself.

Now Mail has taken the trend setter crown, because iTunes 5, released today, sports a blue-tinted side bar that looks very familiar. Just like the mailboxes sidebar in Mail, in fact. You can bet that more of these flatter, simpler sidebars are just round the corner; I wouldn’t be surprised at all if the Finder’s own Sidebar and iPhoto’s Sources sidebar adopt this look in the near future.

And it looks like iTunes is the biggest deal as far as Apple is concerned. Look at a screenshot from the apple.com/itunes page today: iTunes promotion is huge. The iPod nano gets a decent size spot. But you really have to look around before you see the tiny little graphic telling you about the ROKR phone.

Screenshot of iTunes web page

Among other things, iTunes 5 can sync contacts with Outlook, or contacts and calendars with Outlook Express. Apple is going out of its way to make users of iTunes for Windows happy.

Apple wants to push iTunes down our throats because that’s where it makes the money. As a gateway to the Music Store, and as a hugely successful cross-platform application, iTunes brings in cash like no other piece of iSoftware. Nothing like the money Apple makes from hardware, but still a very nice additional income stream. And the long-term potential for selling music - and one day, we all suspect, TV shows and movies too - makes it more valuable still.

But what does Apple get from ROKR phones? Motorola manufactures them; Cingular (and in the UK, O2) sells them. Everyone has to take a cut along the way. I reckon the ROKR is a much bigger deal to Motorola than it is to Apple.

What I like most about today’s announcements are the possible future directions they offer.

I like the lessening of the brushed metal and the flattening of the interface widgets in iTunes. Perhaps we might see this simpler approach elsewhere in OS X.

I like (in fact, I doubt there are many who don’t like) the incredible slinkyness of the iPod nano. In a few years, even the “big” iPods will look like this, and include an order of magnitude’s more storage space.

And in some ways, I even like the ROKR. It is, as the cellphone retailers will be delighted to remind you, the “first phone with iTunes,” but I don’t think it will be the last. I have no doubt that eventually, iPods and cellphones will merge into one tiny device (perhaps one that looks a bit like the nano); we just have to go through some early iterations, and this is the first of them.

The queue for drooling over the nano starts here