Hi, I tried several stuff (I'm in 2.0 with an Iphone 3G) and at the beginning nothing was working (nor Quicktime, nor Audacity) to change the "ringtone" for the "new mail".
I tried MP3->AIFF->CAF with QT, nothing (the sound wasn't played).
I tried with Audacity (MP3->CAF), nothing.
Then I had a hunch and I rebooted the Iphone, then (miraculously?) I was able to hear the .caf I uploaded earlier, the one I made with QT...
Just to be sure I made a test:
I loaded a MP3 with QT, I selected a part of it and I exported it in AIFF (PCM linear, Stereo, 44khz, quality 100%, 16 bits) as "test.aiff".
I renamed in SSH the previous /System/Library/Audio/UISounds/new-mail.caf file to /System/Library/Audio/UISounds/new-mail_spider.caf.
Then I renamed on my computer the test.aiff to new-mail.caf and I upload it to /System/Library/Audio/UISounds/.
I was already on the settings/sound tab on Iphone and I tried to play the new "new mail" ringtone... Surprise surprise, I got... the Previous ringtone I made :lol:
Ok I quit settings, and come back to settings/sound, this time, I got no sound, ok

I reboot the Iphone and this time I got my "new" ringtone.
Then I made a stupid test, I had the file /System/Library/Audio/UISounds/new-mail_spider.caf remember?
So I renamed the file ne/System/Library/Audio/UISounds/new-mail.caf to /System/Library/Audio/UISounds/new-mail_fax.caf and I renamed /System/Library/Audio/UISounds/new-mail_spider.caf to /System/Library/Audio/UISounds/new-mail.caf
And I tried to play it in Settings/sound.
Surprise, this time I could hear the good ringtone without rebooting...
So basically it's working fine with QT, you just have to reboot in worst case scenario.
If someone could confirm it.