Some years ago, if you wrote in C and some subset of C++ and used a sufficient number of platform abstractions (via SDL or whatever), you could run on every platform an indie could get on - Linux, Windows, Mac OS of various versions, obscure stuff like BeOS, and the open consoles like the GP2X and post-death Dreamcast. If you got a contract for a closed platform at some point, you could port your game to that platform with "minimal" code changes as well.

Today, indie developers must use XNA to get on the Xbox 360 (and upcoming Windows phone); must not use XNA to work anywhere else but Windows; until recently had to use Java on Android; Flash doesn't run on phones, HTML5 doesn't work on IE. Unlike e.g. DirectX vs. OpenGL or Windows vs. Unix, these are changes to the core language you write your code in and can't be papered over without, basically, writing a compiler. You can move some game logic into scripts and include an interpreter - except when you can't, because the iPhone SDK doesn't allow it, and performance suffers because no one allows JIT.

So what can you do if you want a really cross-platform portable game, or even just a significant body of engine and logic code?

Is this not a problem because the platforms have fundamentally diverged - it's just plain not worthwhile to try to target both an iPhone and the Xbox 360 with any shared code because such a game would be bad? (I find this very unlikely. I can easily see wanting to share a game between a Windows Mobile phone and an Android, or an Xbox 360 and an iPad.) Are interfaces so high-level now that porting time is negligible? (I might believe this for business applications, but not for games with strict performance requirements.)

Is this going to become more pronounced in the future? Is the split going to be, somewhat scarily, still down vendor lines? Will we all rely on high-level middleware like Flash or Unity to get anything cross-platform done?

tl;dr - Is porting a problem, is it going to be a bigger problem in the future, and if so how do we solve it?