Not sure if they still use a "rational relationship to a legitimate state interest" test, but I can assure you that if `push comes to shove', and another developer complains that some stubborn homeowners are `standing in the way of progress', you will see a federal appeals court enforcing Kelo. That stuff you cited from the majority opinion was just window-dressing.
And that's why developers are ecstatic and so many middle-class Americans are unhappy with them.
There's nothing to enforce in Kelo. The USSC just said that the Takings Clause of the Fifth Amendment no longer protects private property from taking for nonpublic use. It doesn't mean the States must ignore the protections as well, just that they are not bound by it anymore.