The prototype of the cellular MacBook Pro holds a few secrets. Firstly, the antenna is detachable: it is placed in a channel along the screen and is magnetic, allowing easy removal. More interestingly, the connector for the SIM card slot appears to be present on commercial machines without being soldered. This is noteworthy because the (vast) majority of internal 3G modems in Mini Card format (PCI-Express) do not include SIM card readers and assume that the computer has one internally.
The reason is simple: the Mini Card connector provides dedicated pins for reading the SIM card, and it is simpler to integrate the reader into the computer itself than onto the card. In practice, installing a SIM card reader in a MacBook Pro via the residual connector and using a Mini Card 3G card (readily available on eBay) should allow one to assemble a MacBook Pro 3G if willing to sacrifice Wi-Fi.
Mac OS X
Power Mac G3