2001 a Rounded Corner Odyssey
Geeks and Geekesses,... I give you the HP/Compaq TC1000/1100 launched in 2002. Somewhat pre-dates all modern devices by a fair margin. https://en.wikipedia.org/wiki/HP_Compaq_TC1100
Also, similar shaped tablets can be seen in 2001 A Space Odyssey released in 1969.