I made the following test:

#-------------------------------------------------------------------
require 'thread'

mutex = Mutex.new
c_produce = ConditionVariable.new
c_consume = ConditionVariable.new
full = false
t1 = Thread.new do
  while true
	 mutex.synchronize do
		c_produce.wait mutex while full
		print 'F'
		full = true
		c_consume.signal
	 end
         #sleep 0.001
  end
end
t2 = Thread.new do
  while true
	 mutex.synchronize do
		c_consume.wait mutex while !full
		print '.'
		full = false
		c_produce.signal
	 end
  end
end
t1.join
t2.join
#-------------------------------------------------------------------

This should be a classic producer/consumer problem solution. The
output:

F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.F.deadlock
0x2b68300: sleep:-  - c:/ruby/projects/threads2.rb:9
deadlock 0x27bc74c: sleep:J(0x2b68300) (main) - c:/ruby/projects/
threads2.rb:27
deadlock 0x2b68274: sleep:-  - c:/ruby/projects/threads2.rb:20
c:/ruby/projects/threads2.rb:20: Thread(0x2b68274): deadlock (fatal)

What is wrong with the code?

>ruby -v
ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-mswin32]

Also, uncommenting a sleep from above makes the deadlocks disappear
and everything works rather fine - is this just a matter of tweaking
the probability? It ran for quite a long time (several minutes),
that's why I doubt this, but hey - I just might be completely wrong.
Any ideas?