A friend has a half-full jar of raspberry jam. The jam was left on the
counter for a day, then put in the fridge. The jar was never turned
upside-down, but the jam has somehow migrated to the top half of the
jar. So the bottom half of the jar is empty, and the jam is suspended
in the top half.

How did this happen?

Apparently your friend has changed his world-view. 
He has switched from an optimist to a pessimist, and the jar that was
once half-full is now half-empty. Go, be a true friend and try to
convince him that things aren't so bad.

Likely the jam started to ferment near the bottom of the jar. The
micro-organisms released carbon dioxide or some other gas that pushed
the jam to the top of the jar like a piston. If the lid had a tight
seal, the jam would only have been able to rise part way.   Neil

The jam became less dense than the air.

No really, your friend or someone who had access to the jam flipped
the jar upside down, and hit it to cause the jam to end up at the top.
 Then the jam was flipped up-side-up, and did not fall before you saw
it.

Are you a student? Fermentation? Either that or it was making a bid for freedom.

To Guzza-ga:

No, I'm not a student. Just trying to solve the mystery of the jam...

I'll vote with the fermentation crowd, but if you can get toothpaste
to behave similarly, you're onto something!

Just remembering my student days when mysterious food related
phenomena were the norm :-) But you got me wondering, so I asked
several people today and it was a new one to all.

Best

I don't think fermenation explains it - especially if it's store
bought jam.  If it's some homemade brew - then there's no telling what
could be in it and it could be possible.  But I imagine that any
commercial jam manufacturer ensures that no significant amount of
fermentation will occur if a jar sits around for a while.  How many
"upside down" jars of jam do you see in the grocery store?

Even *if* fermentation occurred in the jam to a significant amount,
there is no reason the jam would creep to the top of the jar.  The
produced CO2 from fermentation *is* a gas and is therefore less dense
than the jam, and hence - would not be able to displace the jam from
the bottom of the container.

Another reason that jam would not ferment is that jam also goes by
another name: preserves.  This is because early in the history of jam
invention - it was found that if you add a lot of sugar to something -
it tends not to spoil.  Thanks to (not so) modern day biological
science - we now know that this is because of phenomena relating to
the semipermeability of cell membranes and equilibration of chemical
potentitial, i.e. osmotic pressure.
See the following link:
http://www.newton.dep.anl.gov/askasci/gen01/gen01493.htm

A previous poster mentioned the likelyhood of it having been turned
over, without you knowing, and than put right side up again.  I think
this is the most logical answer, particularly since at lower
temperatures with jam (as with most fluids), the viscosity will be
much much higher (i.e. the colder temperatuer makes it thicker).  And
it is the high viscosity of the jam in the refrigerator which prevents
it from moving to the bottom of the jar, not the formation of CO2 from
fermentation.

I know this one, because the same thing happened to my nephew, the jam
migrated to the top of the jar, then out and across the room, then all
over my nephew?s mouth and face, it appears that jam likes to attack
innocent children.