tranny slipping

[rhughes@domain.elided]

This morning, while pulling away from a stoplight in mid-commute, my engine
revved way up and I could only very slowly accelerate.  The engine didn't
seem to be transferring power to the drive train.  I pulled over and, moved
the shifter through park, reverse, and back to drive, and pulled away.  It
was no better, but I was able to get on the freeway and eventually get up
to speed.  The tranny did shift, but only very slowly, and my rpms were
really screaming.  I got off the freeway and limped to a 7-eleven.

I checked the ATF dipstick and it was dry.  Put in two quarts of ATF, and
got a full reading.  Pulled out and drove the rest of the way to work.
Everything seemed fine -- the tranny didn't even clunk on shifting like it
used to.

The obvious conclusion is that I was just way low on ATF.  But why would
that happen so suddenly?  I didn't notice any major drips or gushes of ATF
under the tranny.  I've got the standard minor leaks all over the place, so
maybe the fluid level finally got low enough that it couldn't transfer
torque?

Does this sound like the right conclusion?  Is there anything else I should
check?  Is there anything else that would make the tranny act that way?
Any help is appreciated, as I know nothing about transmissions.

BTW, I drive a 75 scout II w/ auto (I assume it's a 727).

rob