As of May 2010, there is no way to directly save an mp3 attachment (Ringtone) from email or MMS on the Palm Pre/Palm Pixi device. You can add the ringtone to you phone using the one of the following methods:
Download to PC:
First you need to download the ringtone to your PC. Then connect your Palm Pre/Palm Pixi to your PC and select USB Mode. Then just drag & Drop the file from your PC into the /Ringtones folder on your Palm Pre. Safely remove the phone, then set the ringtone.
Send to Email:
Once you get that email, open it on a PC. Save the attachment on your PC, then connect the Palm Pre and choose USB Mode. Then just drag & drop the file from your PC into the /Ringtones folder on your Pre. Safely remove the Palm Pre/Palm Pixi, then set the ringtone.
It's several steps to take, but currently there is no way to directly save an mp3 attachment from email or MMS on the device. You could request this option at www.palm.com/feedback where the developers monitor for customer requests of new features.
You can use the following method to download ringtones for your iPhone: