Development-wise that problem has mostly been solved. In the past cross-platform game development was done as separate projects, often handing off "porting" the game to a platform to an external team, even. Today it's more common to use engines (such as the unreal engine) which support all the major platforms (PC, 360, and PS3 typically, for first-person, high graphic-load style games), and then do the development for all versions concurrently. Typically you'd just have your regular builds set up to produce versions for each platform as a matter of routine, and you'd have a consistent testing experience for each. You'd still end up with customizations and tweaks for each individual platform but for the most part all of the hard work of targeting and optimizing for each platform is automated.
Just for another perspective, my experience doesn't really align with this. From my (biased) perspective, this sounds like "in theory" as opposed to "in practice" so I'm honestly curious if you've managed to get away from having to really deal with cross-platform differences.
I know that in theory you use a cross platform engine that hides away all the messy details from you.In practice, I've found the PS3/360 have quite different CPU/memory architectures and you start dealing with the reality of that sooner than you'd expect. Whether that requires manually down sizing certain swaths of textures or writing code that will run on the SPU, there's quite a bit of manual work involved.
Also, most game studios tend to roll their own tech which generally requires at least a small cadre of people manually targeting and optimization for each platform.
