Is there a custom define for Windows 8?
Can I expect our games to work on Windows 8 as in Windows 7?
Its UNITY_METRO.
If you are talking about Windows Store apps they are a lot different from desktop stuff so you need to be a lot more careful how the content is or how complex the project is so you don’t hit problems during review/approval process.
Your games built for PC Standalone will work fine on Windows 8.
Of course Windows Store Apps is seen as another platform and exclusive to Windows 8, and there are other things you need to factor into the development like testing every aspect of the pesky snapped mode and making sure your game handles it gracefully.
Then if you’re doing any sharing you need to make sure it’s done the ‘correct’ way for a Windows Store app, i.e using the Metro API’s and not doing your own. I’d suggest reading through the Windows Store Apps docs to see what can, can’t, should/shoudn’t be done.