More than 100 IBM employees occupy Apple’s Cupertino, Calif., campus helping build iPhone and iPad apps for IBM customers such as Citigroup Inc., Sprint Corp. and Japan Post Holdings Co.

Things are looking different inside IBM, too. Once a company of blue suits, Wintel PCs and BlackBerrys, Big Blue is on track to become the world’s largest corporate user of MacBooks. On Wednesday, the company will apply lessons it has learned to introduce a new service intended to help other companies adopt Macs.

This is an important relationship for both companies. It’s really bizarre how things change so dramatically over time though.