Is sub critical plutonium "safe" to handle? Apparently, in Los Alamos scientists handled sub critical masses of plutonium (for example the demon core) with little or no protection. Richard Feynman and others mentioned that plutonium spheres were warm to the touch, which seems to imply that they actually touched them. Slotin (one of the victims of the demon core) performed experiments wearing blue jeans and cowboy boots.
Are certain plutonium isotopes safe to handle (as long as they don't go critical) or is it just that people back then didn't know any better?
 A: Please be aware that plutonium cores are supposed to be plated with another metal (nickel or silver, if my memory serves me right). Machining plutonium is very hazardous and is done with remote manipulators, since it increases risk of inhalation.

Source: http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@na+@rel+plutonium,+radioactive :

Absorption through the skin can occur through occupational exposure. Experiments show that the skin is an effective barrier and the percentage absorbed /seldom/ exceeds 0.05% for intact skin.
  [Seiler, H.G., H. Sigel and A. Sigel (eds.). Handbook on the Toxicity of Inorganic Compounds. New York, NY: Marcel Dekker, Inc. 1988., p. 724] PEER REVIEWED


Source: Plutonium ANL FactSheet Oct 2001


Plutonium metal. Plutonium isotopes are primarily alpha-emitters so they pose little risk outside the body. Here the plastic bag, gloves, and outer (dead) layer of skin would each alone stop the emitted alpha particles from getting into the body.
What Happens to It in the Body? When plutonium is inhaled,
  a significant fraction can move from the lungs through the blood to
  other organs, depending on the solubility of the compound. Little
  plutonium (about 0.05%) is absorbed from the gastrointestinal tract
  after ingestion, and little is absorbed through the skin following
  dermal contact. After leaving the intestine or lung, about 10%
  clears the body. The rest of what enters the bloodstream deposits
  about equally in the liver and skeleton where it remains for long
  periods of time, with biological retention half-lives of about 20 and
  50 years, respectively, per simplified models that do not reflect
  intermediate redistribution. The amount deposited in the liver and
  skeleton depends on the age of the individual, with fractional
  uptake in the liver increasing with age. Plutonium in the skeleton
  deposits on the cortical and trabecular surfaces of bones and slowly
  redistributes throughout the volume of mineral bone with time.
What Is the Primary Health Effect? Plutonium poses a health hazard only if it is taken into the body because all
  isotopes but plutonium-241 decay by emitting an alpha particle, and the beta particle emitted by plutonium-241 is of
  low energy. Minimal gamma radiation is associated with any of these radioactive decays. Inhaling airborne plutonium
  is the primary concern for all isotopes, and cancer resulting from the ionizing radiation is the health effect of concern.
  The ingestion hazard associated with common forms of plutonium is much lower than the inhalation hazard because
  absorption into the body after ingestion is quite low. Laboratory studies with experimental animals have shown that
  exposure to high levels of plutonium can cause decreased life spans, diseases of the respiratory tract, and cancer. The
  target tissues in those animals were the lungs and associated lymph nodes, liver, and bones. However, these
  observations in experimental animals have not been
  corroborated by epidemiological investigations in
  humans exposed to lower levels of plutonium.
As a note, the common myth that
  plutonium is the “deadliest substance known to
  man” is not supported by the scientific literature. It
  poses a hazard but is not as immediately harmful to
  health as many chemicals. For example, for
  inhalation – the exposure of highest risk – breathing
  in 5,000 respirable plutonium particles, about
  3 microns each, is estimated to increase an
  individual’s risk of incurring a fatal cancer about
  1% above the U.S. average “background” rate for
  all causes combined.)

EDIT: As an aside: I recommend reading Eileen Welsome's The Plutonium Files: America's Secret Medical Experiments in the Cold War to get some idea of what early plutonium health safety experiments really meant (e.g. injecting a solution of plutonium salt into a patient's leg).
