There are a number of Windows-based laptops that have an integrated 3G wireless card inside, giving them the ability to connect to the Internet from pretty much anywhere. More recently Samsung and others released the first Chromebooks, based on Google's Chrome OS. Those notebooks are dependent on a 3G connection in order to work well. Apple has never released any of its MacBook notebook PCs with such features. However, that doesn't mean Apple didn't work on such a product.
As first reported by MacRumors, a prototype MacBook Pro with a 3G card and antenna is now on sale at eBay. The prototype was supposedly made way back in 2007 when having a 3G connection for a notebook was more of a luxury. According to the eBay description, the current owner originally bought the MacBook Pro via Craigslist, but as more of a parts machine purchase. The examination of the parts showed off the 3G card and antenna. While the MacBook Pro has been restored to working order the eBay listing says the 3G data connection doesn't work.
This is definitely more of a curiosity than anything else, but it does show that at least at one time Apple tried to put in 3G connections in its MacBook Pros. Of course the current version of the iPhones and Apple's iPad have 3G connectivity. There's no word as to why Apple chose to abandon this project. The eBay auction still has about four days left with the current high bid, as of this writing, set at $4,550.