Mobile PC has a great article (with pictures) outlining the evolution of mobile computing:

1975
IBM 5100 Portable Computer
The first computer with a built-in display, this 50-pound monster was swept under the rug after the PC came out in 1981.

1981
Osborne 1
Adam Osborne's labor of love was an overnight success ... and an overnight failure. Today it is remembered fondly as a pioneer in portability.

1982
GRiD Compass 1100
The first mobile computer with a folding screen, the GRiD Compass was a coveted survivor for more than a decade.

1982
Epson HX-20
The world's first "laptop," designed as a slate with no folding display.

A few months ago, one of our retiring specialists brought back twenty years' worth of luggable, portable, and laptop computers. Looking at the twenty-five plus pound original machines with their two 5.25" floppy drives, I'm guessing we shouldn't complain too much about carrying six and eight pound computers around.