I was watching a program on TLC about space junk. Apparently radioactive payloads have been going into space at least some 26 years before the shuttle explosion. And it's also reentered the atmosphere a couple times.

A Russian craft used plutonium to keep its avionics warm. Bound for Mars, it fumbled on takeoff and wound up stuck in low earth orbit. It circled Earth a few times and reentered the atmosphere. The Russians claim it landed (harmlessly...) in the pacific ocean.

A Pu laden US experiment from the 60's called SNAP 9A was part of a malfunctioning Navy spacecraft that said oops and sprayed itself throughout the atmosphere. It was said that everyone now carries a small quantity of plutonium in them due to this event.
Which was interesting but a bit weird. I can think of a couple ways Pu can bridge a generation, but the show didn't go into it. NASA claims SNAP burnt up in the upper atmosphere, "as designed", and said nothing about mass plutonium poisoning. Intriguing!

NASA has launched 38 of these RTG's (radioisotope thermoelectric generators) so far. Here's another one:

"An experimental nuclear reactor power system, the SNAP 10A which used thermoelectric power conversion, was launched by the United States in 1965 and worked satisfactorily for 43 days until shut down. It is now in a very high orbit where it will remain for hundreds of years." ---nasa.gov
Neat. I hope they put a "Plutonium On Board!!!" placard in it's rear window, to warn off future space travelers from Earth or otherwise.

The modern US probe Cassini carries around 72 pounds of plutonium. In August of 1999 it performed a gravity assist swingby in order to speed it's quest to Saturn. It flew within 725 miles of Earth, and (according to NASA) would have vaporized 32-34% of the Pu into the atmosphere if it accidentally reentered. That's around 25 pounds of plutonium. Originally it was to fly within 312 miles of Earth, but the figure quickly grew due to public outcry.

A space show on TLC (Didn't get the title, clicked in halfway through)