Windows phone 8 will probably have a larger installed base than windows phone right of the bat, so cool apps like gmail, google maps etc will soon be available on that. How easy is it for app developers to make their windows 8 apps available on windows phone 8 platform?
It depends on the complexity of the application, but if you are writing a "Metro UI" application on Windows 8 then porting may be relatively easy:
If you have a game written in C++/DirectX or an app that works inside an IE control, it should be easy to port to Windows Phone 8. For everybody else going from Windows 8 to Windows Phone 8, you've got a whole lotta re-imagining and re-coding to do.
Until Microsoft publicly releases details on how to write applications for Windows Phone 8, there can only be speculation; What is known is that there are some differences between Windows Store style applications, and Windows Phone Apps, that mean developers will have to download additional tools to target Windows Phone devices.
It has been said that the vast majority existing Windows Phone Apps will continue to work on Windows Phone 8 (so it may actually have more apps at launch) - there are caveats if developers have tried to be sneaky to expose something they shouldn't have access to.
Until the SDK formally releases, deep details aren't available. But you will not be able to directly take an executable binary from Win8 to WinPhone8. If you're looking at doing layers that are portable between Win8 and Windows Phone, spend some time studying how to use the Portable Class Library Project in Visual Studio 2012. Key piece of portable. Here's a link to get you started - http://msdn.microsoft.com/en-us/library/gg597391.aspx . Pay special attention to the "Supported Features" chart.
Here's a primer from MSDN Mag - http://msdn.microsoft.com/en-us/magazine/hh335063.aspx .
This channel9 video has a ton of resource - http://channel9.msdn.com/Shows/Visual-Studio-Toolbox/Visual-Studio-ToolboxPortable-Class-Libraries .