White smoke from engine

I took my car in for an oil
change/servicing about a week ago.
I noticed that once, when starting up,
the car seemed to shake more than
usual for a second or two, but then
was fine.  I had no other problems -
driving about an hour a day for work
- until today.

Today I drove about an hour and a
half north, picked up a friend, and on
a side street while going downhill we
both suddenly noticed a burning
smell and saw white smoke coming
from the engine.  The heat gauge
said the temperature was fine.  I
pulled into a parking lot and popped
the hood - but we couldn't see
anything smoking - or smell anything.  
I turned the engine back on - and
smelled the burning-smell again from
the vents - but didn't see any more
smoke or smell anything from the
engine outside the car.  I drive back
to my friends - about 5 minutes, but
including a steep hill - and the
problem did not recur.

Any idea what the problem likely is?  
Is it probably safe to drive home, so I
can see a local mechanic and not
have to spend the night up here?  
What precautions should I take?