What is the maximum filename length in older versions of Windows, and
in Windows XP, Linux and 64-bit Linux (stuff built on the new 2.6
kernel?), and current Mac OSes? Are there any limitations based on
hardware?

In general, IIRC, the max filename is ~255 characters. This limitation
is due to the FileSystem, not (necessarily) hardware.

Consider: A filesystem (FAT, NTFS, UFS, etc...) must handle the
overhead required to handle a long filename. Though it's possible to
create a filesystem with arbitrarily long filenames, the benefits are
generally lost in the space required to store the filename, as well as
the user's ability to both effectively understand the content by
filename and display a list of files. On one hand, 8.3 wasn't too
useful for identification in DOS, but the disk drives and hard drives
were originally small, if available. On a 720K 5.25" floppy, one can
see that arbitrarily long filenames can eat up space fairly quickly.

But, the limit is based upon the filesystem. If *I* were to consider a
filesystem with arbitrarily long filenames, I might consider actually
having a numeric index tag for the files, which would be used both for
data and a LFN table. If I had a guess, I would believe that Windows
9x actually used this or a similar method (see that 8.3 support is
still implemented) to provide LFN support and still maintain backwards
compatibility with 8.3 DOS without having to have a conversion
process. Actually, I would be interested to know what possibilities
could exist that hardware is a limitation, if at all. I just don't see
it. Especially if the data can be in Terrabytes.

Referencing this page: http://my.execpc.com/~geezer/osd/fs/

FAT12/16: <8 characters>.<3 characters>
VFAT/FAT32/NTFS: 255 characters
ext2 and ext3: 255 characters
(http://lists.samba.org/archive/rsync/2002-June/002862.html)
Mac OS 9: 31 characters (http://keepitsimple.confusticate.com/projectmac.html)
Mac OS X: 255 characters (http://www.webmasterworld.com/forum49/76.htm)

64-bit Linux uses the same filesystems as 32-bit Linux, so the
limitation is the same. The only hardware limitation that exists is
the size of a hard drive. You can't store a 2 gigabyte long filename
on a floppy disk.