After a little fiddling around, I have figured out why, and I will step you through how to change this.
Here's the short version:
Edit > Preferences > Import Settings... > Import Using: (drop-down menu) > AAC Encoder > OK > OK > Right-click > Create AAC Version
Here's a more detailed walkthrough with images. Things are labelled in green to make things easier to locate. Click on the thumbnails to enlarge the images.
|Initially, this is all that is displayed when you right-click.|
|Click Edit > Preferences|
|Once that appears, click 'Import Settings...'|
|When that comes up, click the drop-down menu labelled 'Import Using:'|
As you can see, mine is/was set to MP3 encoder.
You need to change it to 'AAC Encoder'.
|'Create MP3 Version' (or WAV Version, AIFF Version or Apple|
Lossless Version depending on what you had it set to) should
now read 'Create AAC Version'
I think iTunes 10's default import setting is 'MP3 Encoder' for most, if not all people, whereas I think (not sure) that any version prior to 10 has it's default set to 'AAC Encoder', which would explain why a) so many people have had issues, and b) why I've never had this problem before - all my other ringtones were done in earlier versions of iTunes (I haven't made one for ages).
Now everything in that tutorial from where it says: " Now click “OK” and then right click on the song again, and select “Create AAC version” to create a new version of the song with the 30 second interval you specified." should make more sense.