I believe he was the last Labour prime minister before Blair, with Thatcher and Major as the intervening placeholders.
So this is important news, no matter how many Kos/sacks/monauts/sites/ have heard of Jim Callaghan or not.
It would be sort of as if his fellow "JC" Jimmy Carter died, he the last Democrat before Clinton.
See, e.g., "Former PM Lord Callaghan dies aged 92" in the Guardian today at http://politics.guardian.co.uk/labour/story/0,9061,1446242,00.html , with tidbits like
"He was haunted by the infamous headline "Crisis, what crisis?", attributed to him when he returned to a freezing, strike-ridden country from a sun-drenched summit in Guadeloupe. ...
A few months later, on March 28, 1979, the government lost a no-confidence vote by 311 to 310."
As close as Florida 2000. Oy.
RIP JC. Without you, the Sex Pistols would have had nothing to write about, but we like you anyway...