Is this just me or is 'ri fork' not being helpful?
$ ruby -v
ruby 1.9.0 (2004-09-10) [i686-linux]
$ ri -T fork
More than one method matched your request. You can refine
your search by asking for information on one of:
Thread::fork, Process::fork, Kernel#fork
$ ri -T Process::fork
---------------------------------------------------------- Process::fork
Process.fork [{ block }] => fixnum or nil
···
------------------------------------------------------------------------
See +Kernel::fork+.
$ ri -T Kernel::fork
Nothing known about Kernel::fork
$ ri -T Kernel#fork
------------------------------------------------------------ Kernel#fork
Process.fork [{ block }] => fixnum or nil
------------------------------------------------------------------------
See +Kernel::fork+.
It looks like the problem is a missing 'call-seq' in process.c, for rb_fork.
(Apologies if this has been corrected in the last week.)