Are they really radioactive?
Yes! The uranium-dioxide in the glass is slightly radioactive. You can actually measure it with a Geiger counter. It's a safe level of radiation...but just to be safe, don't eat the marble.
Here is the reading from a bunch of larger marbles:

Why would someone want to put radioactive material in glass?
The glass is actually called Vaseline glass. By adding the uranium-dioxide, it gives the glass a green-ish color (hence the Vaseline name). For more information on Vaseline glass, be sure and check this link.

Ok. So it's radioactive. Anything else I should know?
Yeah! Wanna see canadian pharmacy something cool? Look at the marble under a black-light, or UV light. It will glow a brilliant green color. This verifys the uranium-dioxide content of the glass.
Normal light: UV light:

Why is this your Geocaching signature item??
I wanted to find something simple, but interesting. I think by handing these things out, they can become interesting conversation pieces.

-fractal