A blog post by Dave Curran on the SD2PET, a SDCard adapter for the Commodore PET which is now available for purchase.
I have to say, solid state storage very much revitalised my interest in running old machines. It turns out that the right kind of SD Card speaks a rather simple serial protocol which can be bit-banged from any kind of parallel port, using just a few pins.
If you use an Arduino, it has pre-written libraries for SD card access as well as FAT filesystem access. Makes a nice, simple project.