Hi. I Just stumbled on this forum.
I have made the modification using the DVD slot, My need was not a 3G card, but a PKI card reader.
Since i was mounting a HUB inside i decided to put in a 3G card and a EVDO card at the same time ( I live in China, so I need alternatives :0) )
Check out the modification here: http://blog.prowlan.com/?p=1106
It can be done much nicer, but I didn't want to put too much work into it before i found out if i liked the mod… But now i think I am going to make the final changes. I will try to locate an alternative USB port in the mac and that way not waste my 3'rd external port.
This mod would by the way work on any mac with an optical drive.
For the people who don't like soldering, there is both 3G and EVDO cards available here in china that sits flush in the Express card slot… e.g. there is no antennas sticking out unless you decide to get better coverage and pull out the small antennas on the cards… my experience is that it works without doing this in most of china :-)