I picked up a six pack of JuiceBoxes from Overstock. I have a fair bit of experience with ucLinux on ARM systems from a project I did at work. I've got thousands ucLinux/ARM boxes in users hands but my devices have LCD screens or audio out, they're just networking devices with wifi, so JuiceBoxes are interesting to me. Unfortunately most of my skills are software/config oriented and not hardware hacking so I killed my first JB. Good thing I got the six pack! I was a bit too aggressive desoldering the JB cartridge socket off the board and ended up with a device that won't boot anymore. Oh well. ;)
On my second attempt I have a device working with a SD memory socket. I used an ALPS socket, I think it's the same one another person on this board mentioned, I can highly recommend it and at $1.70 per unit its hard to beat. The specific one I used can be had from Mouser.com, which doesn't have a minimum order but does have realistic shipping prices.
The model I used is SCDA1A1301
You can see a diagram of this socket and similar models in this PDF:
ALPS part # SCJB2B0101, fig A on that PDF also looks promising at only 2.4mm thick.
I've got some pictures of my installation here:
This installation is pretty clean but the card does still stick out a little bit due to the length of the card mainly, the socket doesn't add too much. It is very nice to use a real SD memory socket that functions just like one you'd find in a digital camera or PDA. At ~$4 for a JB plus $1.70 for the socket, add some memory and you've got a nice little MP3 player for the kids, in fact when I got this first one working I immediately had two of my kids asking me when I'd have theirs ready!
Next I'd like to take a shot at getting Linux going, I don't have any JB carts but I did pickup an ALPS xD/SD combo socket as well. I don't think I have anything I can use to program xD in the raw so I'm probably going to have to figure out the JTAG @ 3.3 volt thing.