Did a little research, and apparently the cocoa mulch warning is quite
real.  Below is a link to an articla writte by Sandra Mason, Unit
Educator: Horticulture & Environment for the University of Illinois,
Champaign.  Excerpt from article (which may be found, in its entirity at:
http://www.urbanext.uiuc.edu/champaign/homeowners/020608.html ) follows:
 
" As Wiesbrook and Gwaltney-Brant reported, the amount of methylxanthines
in cocoa hulls is substantial at 255 mg/oz.  And that's just the
theobromine; no data was available for caffeine.  In comparison, milk
chocolate has only 64 mg/oz of methylxanthines, and less than 1 oz of
milk chocolate/lb (2 oz/kg) is potentially lethal to dogs.  So 65 oz (4
lb) of milk chocolate would be potentially lethal for a 65-lb dog.  But,
if she were to eat cocoa-hull mulch, it would only take about 2.25 oz to
produce mild signs and 12 oz to be potentially lethal.  Of course, these
amounts would be much less for a smaller dog.
 
"Some manufacturers do include a warning statement on the bag.  If you
have an older dog that isn't all that inquisitive, cocoa mulch may not be
a problem, but if you have a young dog that tends to dig and chew, you
may want to consider different mulch.
 
"For more information, call your local veterinarian or animal poison
control center.  You can reach a link to the center by clicking on
http://www.aspca.org.  Also at this site is an article on chocolate
intoxication http://www.aspca.org/apcc/toxbrief_0201.pdf that gives
detailed information on clinical signs and treatment, as well as an
example for calculating the methylxanthine dosage.  At the above Web
address, you'll also find information on protecting your pet from
pesticides and fertilizers, as well as a list of plants that are toxic to
pets.  Thanks to Wiesbrook and Gwaltney-Brant for alerting us to this
potential problem.  "
[Posted in FML issue 4169]