Less than a week after they were announced, ringtones are now available for purchase through the iTunes Store, and several new features have popped up in iTunes to take advantage of the functionality.
First of all, you can now search through your current iTunes library to search for purchased tracks that are eligible for ringtone creation. While viewing your iTunes library. Go to the "View" menu and select "View options...," then check the box next to Ringtones.
Click the "OK" button, and you'll be presented with the following dialog box:
You can now click on the ringtone category icon at the top of your music list to organize tracks by whether or not they can be made ringtones. Click on the ringtone icon next to a track to bring up the ringtone editor and start creating a tone. Unfortunately, for many already purchased tracks, the ringtone editor (which slides up from the bottom of the screen) kept disappearing during the "Accessing iTunes Store" phase. However, with newly purchased tracks, we were able to click through, agree to the iTunes terms of service, then properly edit a track and create a ringtone.
Ringtones can also be purchased from the iTunes store once a track itself has been purchased, though many ostensibly available tracks carried a grayed-out, non-clickable ringtone icon in our testing.
It looks like (as of late Monday night) Apple is still rolling out changes to the database, as new tracks are being found as eligible in our music libraries.
You'll also notice that Ringtones entry has been added to the left-hand pane under the "Library" section. This will show any ringtones you've created, as well as any items you've manually added via this trick