I think it's largely due to contracting issues. I don't think Apple want to be tied into contracts for mobile broadband plans.
Their notebooks are already rather largely squashed together, doubt theres much room in there for a 3G card, especially with the enclosure being aluminum, the signal would just... suck quite frankly.
And TBH IMO, when sites like dell have the setting "Yeh yeh yeh get this super awesome speedy mobile broadband trial, just pay $99.95 upfront and sign a 36 month contract.." looks sort of... dirty. I think it would ruin Apple's reputation.