I own a Nokia 6102i phone (provided by Cingular). gnokii is a Linux program that lets me interface with the phone. Here are some recipes:

File I/O

gnokii --getfilelist "A:\\predefgallery\\predeftones\\predefringtones\\*"

gnokii --putfile WiiSports.mp3 "A:\\predefgallery\\predeftones\\predefringtones\\WiiSports.mp3"

Ring Tones

Voice mail picks up in 20 seconds, so a ring tone should be about 20 seconds long.

The easiest way to chop an MP3 in Linux is with dd; the drawback is that you need to specify length in KB, not time. To chop an MP3 to be 200 KB long, do:
dd if=Mii\ Channel.mp3 of=MiiChan2.mp3 bs=1k count=200

Phonebook

To make a Phonebook.ldif file from the phone (suitable for import into Thunderbird):
gnokii --getphonebook ME 1 end --ldif > Phonebook.ldif

To add the entries in Phonebook.ldif to the phone:
cat Phonebook.ldif | gnokii --writephonebook -m ME --find-free --ldif
You can specify --overwrite instead of --find-free if you want to overwrite all the entries, but this will lose some data (e.g. speed dial, preferred numbers).

Multimedia

You can get photos like this:
gnokii --getfile "A:\\predefgallery\\predefphotos\\Image000.jpg"
They are 640x480 JPG files. (You can also configure the camera so that it takes pictures at 80x96.)

You can also store files:
gnokii --putfile silly.jpg "A:\\predefgallery\\predefphotos\\silly.jpg"
These show up on the phone in My Stuff/Images. The files don't need to be any specific size; they are autoscaled. GIFs probably also work.

Videos live here:
gnokii --getfile "A:\\predefgallery\\predefvideos\\Video000.3gp"
VLC seems to be able to play .3gp files, but the audio doesn't work.

Audio recordings live here:
gnokii --getfile "A:\\predefgallery\\predefrecordings\\Audio000.amr"
Unfortunately, nothing I know of will play .amr files, but maybe this will help: http://linux.wordpress.com/2006/09/19/convert-wav-and-mp3-files-to-amr-in-linux-using-gstreamer/