New development and new documentation is now (as of November, 2003) being maintained at the new official home of Cells:
Cells on Common-Lisp.Net
There you can also sign up for a mailing list to keep you apprised of new work on Cells and Cello, the Cell-based portable CL GUI.
Cell Basics
Just enough to make cellular code readable and demystify the behavior of cell-powered CLOS instances. The extension is .LISP, so you might have to save to disk before opening it.
Bill's Most Excellent Cells Intro
What can I say? Bill Clementson writes better than I do. A very good place to start. Check out the rest of his site as well for lotsa good stuff on Lisp and more.
There is a reasonably approachable Overview and a sick, intense, impenetrable hence poorly named Hello World. It deliberately walks you through Cell programming errors so you recognize them when you backtrace, and so you know how to avoid or fix them.