It is my understanding from reading the Cricut messageboard that the pen attachment that came with the QVC package will be offered separately later this summer and will fit the original Cricut. No idea on price.
There is an old post on there where one gal used Crayola Pipsqueak (chunkier, not thin) markers in her Cricut. She just put it in the holder for the blade assembly. She did have to secure it with a rubberband, though since the screw is not long enough to fully close in on the marker. She showed pictures of what she had been able to draw. I haven't done this myself yet, but I don't see why it wouldn't work after reading about her success. The Cricut machine is essentially a "plotter" which is used for drawing.
I would like to try it sometime, though.