It appears that at one time Apple was exploring 3G-capable MacBook Pros. eBay seller Abhw752579 has listed a prototype 2007 Santa Rosa MacBook Pro with an integrated 3G antenna built in to the LCD screen. The eBay seller is not the original owner or the machine, but bought it from someone on Craigslist who was selling the laptop "for parts." However, upon disassembling the MacBook Pro, Abhw752579 discovered it contained a red motherboard, which typically signifies an Apple prototype model. Of course the other feature that should have immediately tipped off Abhw752579 that something was out of the ordinary with this particular MacBook Pro was the huge 3G data antenna hanging off the laptop's screen.
As you can see from the image above, the 3G antenna is extendable, and Abhw752579 notes that its underside is marked with "Tyco Proto / #006" in ink. In addition to the 3G antenna, there's also a standard size SIM card slot located underneath the memory cover on the bottom of the MacBook Pro. Abhw752579 was able to get the prototype powered on and discovered that OS X did pick up the 3G antenna, but there was of course no data signal.
Users have been hoping for a while now that Apple will add 3G connectivity to their laptop line and this prototype confirms Apple has at least explored the option. However, after seeing how ugly a MacBook Pro looks with an antenna hanging off of it, I'm glad Apple never brought the laptop to market. Then again, 2007 was a long time ago (in tech terms) and if Apple can build a concealed 3G antenna into an iPad, they can easily fit the same antenna into the existing form factor of today's MacBook Airs and MacBook Pros.