Next time there will be more on the five Gartner-models. The first and maybe biggest problem is system-management. Since you have to manage two or more systems, most of the time from different vendors, instead of one system management will be increasingly difficult. System development itself can be more complex, since you have to develop for more than one environment, often having to work with a great deal of software-legacy. Development will be more of a migration, instead of a great big bang in which you introduce a totally new system at once.
Developers have to integrate different environments from different vendors.
In spite of all the promises of vendors that product X will work with product Y, be prepared for a lot of problems on that point. My name is Coen de Heer.