Definitely possible, and on my list of things to do. It isn't even very hard actually, the controller can already export the queue as a text file. And there's no reason why the server couldn't go into "buffering" mode where it stores the instructions away on the card instead of executing them directly.

I wrote about trying to get this working, and the SPI / motorshield issue on http://forums.adafruit.com/viewtopic.php?f=31&t=22422&p=117621#p117621, a wee hack, but more or less functional I think. This was before I was really using the servo though, and it did cause a problem with the servo, so it might not be completely practical if you require a pen lift.

The real issue is that the libraries that are needed to usefully access the SD card as a drive are pretty big, and memory is already tight on a regular ATMEGA328. I stripped it back to the minimum required to read only from a sd card using a proprietary file allocation format, and it would compile and start ok, but it was pretty close to blowing the stack almost continually, and frequently did. Let me know if you're interested and I'll dig the code out and put it back into the repository in some form.

For standalone drawing, I really think something bigger than a regular arduino is required, so I was eventually using an arduino Mega - actually a DFRobot clone. The SPI interface is on a different set of pins on the mega, so the motorshield no longer inteferes with them.

I never actually got round to using it in anger, since I got sidetracked with the kitting projects, and it needs extra things like buttons to be useful. But I intend eventually to offer the core code as a library, and have a reduced firmware version for regular arduinos (ie the code as it stands now), and an advanced firmware version that will be a bit more clever and do stuff like reading/writing commands from file, and even reading / writing images directly so it can do stuff like work off a camera without having to have a computer attached. Not sure how feasible that is, but have been looking into it.

Cheers
Sandy Noble

Sure, don't see why not, if it's equipped to run the servo (spare PWM pin).

I think there still might be a way to do it, but I admit I stopped looking for it once I realised the SD card library wasn't going to fit into the memory on the ATMEGA328 anyway.