I'm pretty much just waiting on an answer from the OpenMCL developers telling me where we're supposed to be getting MOP functions from. For kicks, I checked Classic MCL this weekend, and it partially works. One of the unbound-cells tests failed, and I didn't feel very inspired to debug it. That does mean it got pretty far through the test suite, though. When I get the answer back from the OpenMCL developers, I'll update the candidate, and ask the couple of you who had problems with it to see if it works. Then, release :-)
Here's the implementations I think Cells supports:
Allegro, SBCL, LispWorks, CMUCL, OpenMCL.
MCL was formerly supported and should not be very difficult to get working again.
What about CLISP? Is it in the same category as MCL, or does anyone use it?