I have this program called Audacity, Im sure some of you guys know of it or use it (its FREE!!) Its just a audio editing program.
All you have to do is import a mp3 or aiff or what ever, then export it as a Ogg vorbis file.

One question though is can you import your itunes playlist?
i think that would be pretty nice for those that use itunes as thier primary audio program.

Here's something you *can* do: make a playlist in iTunes, then select all the tunes in it. Drag them to a different drive, or to your desktop. Use them as the source files for Audacity [load 'em up in Audacity one by one] and export each as OGG.

If you're using a Mac, note that Audacity doesn't open bog-standard AIFF files, but if you're on a PC, the standard WAVs will open in iTunes, even if you move them to a Mac.

I exported a bunch of WAVs to OGG for a pal who plays a different game with me, Operation:Flashpoint. I used my PC for the whole operation.

I'm told, by the Audacity users' list, that there's a way to use text files as control files to batch-process a group of audio files with Audacity, but I've never looked into it. If you're into command-line work, that might be a useful avenue to investigate.

#!/bin/bash

DIRECTORY="~/music/"
CURRENTDIR=`pwd`
cd ${DIRECTORY}
for i in *.mp3
do
NEWNAME=`basename "$I" ".mp3"`.ogg
mpg123 ${i} | oggenc -raw -o ${NEWNAME}
echo${NEWNAME} >> playlist.m3u
done
cd ${CURRENTDIR}

Erm... What? :
you will actually encode something like :
Playing MPEG stream from carol.mp3 ...
Found new ID3 Header
MPEG 1.0 layer III, 160 kbit/s, 44100 Hz joint-stereo

.. and try to feed that to an ogg encoder... *cough*

try something like this instead:

for i in *.mp3;
do
madplay -o wave:- "${i}" | oggenc -o "${i/mp3/ogg}" -
done

If you want to keep metadata, there are nice ways of extracting that as well, but I'm too lazy to type it in right now. id3cp might help you.

And to throw one more in...

Since oggenc accepts more than one file as argument and does the naming automatically (atleast mine does) you can just do an:
oggenc *.wav
in the directory with your files.

(same thing for *.aiff *.au .. )
(this does _not_ work with mp3 or any other fancy formats though )

It's also not too great an idea to convert mp3 or other lossy compression formats to ogg, because ogg is also lossy, which means you will lose the parts that mp3 would lose, and what ogg would lose, so you end up with lower quality than ogg is really capable of. It's especially bad if you start with a low quality mp3 (128 Kib/s or less).

The formats oggenc supports directly (raw, WAV, AIFF, AU, FLAC) are all lossless, and thus good candidates to encode from.