I don't know how it compares to OLPC, but it's basically what you're describing.
The biggest problem however is that it costs as much as a cheap netbook before you include the Raspberry Pi and SD Card. Finally, I'm not one of those people who has to have the thinnest laptop ever, but the inside of the Pitop is cavernous. You can actually store an ethernet cable inside of the laptop! But it's hilariously thick, it feels like a 1990s laptop.
The pi-top is only very superficially what shams93 describes.
[0] https://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery
The product page doesn't give details of the battery, so I assume it's lithium ion rather than lithium ferrous phosphate (i.e. prone to explode)? Can't see what screen technology they're using, but if it's anything like standard laptops it won't be readable in sunlight and will require a power-hungry backlight.
Thankfully modern laptops don't seem to require mercury in their backlights (now that they're LED), so the "non-toxic backlight" feature of the OLPC is no longer necessary.