That matches my understanding.  I think (but am not 100% sure) that
rbus_min_start has to be

  greater than actual physical memory

  perhaps greater than installable physical memory

  less than some device-dependent value depending on some address
  lines not working or something like that

On a Thinkpad 600E with 192 MB RAM, I needed to set RBUS_MIN_START to
512MB.  man cardbus(4) for details.  Interestingly, on a 600 (2645-41U
- older but similar model) with 96 MB, the default rbus_min_start
works.

It would be nice to have the kernel adjust rbus_min_start basd on some
heuristics (if not set by config) to make it more likely to work.  I
have such a (pretty grossly implemented) heuristic in a private source
copy, and have been meaning to clean it up.  Essentially, if actual
ram is < 256MB, then it sets rbus_min_start to 512M.  This fixes my
600E and lets me run the same kernel there and on more modern
machines.

You went to 256M - I wonder if 512M works, or if we need a <128M uses
256M rbus_min_start also.

Actually trying various values would be better, but it seems the cbb
attaches fine and it's only when a card appears that there is trouble
- and that might be too late.

-- 
        Greg Troxel <gdt@ir.bbn.com>