Please help me on this. Is it possible to deploy a WSP solution developed in SharePoint 2010 to SharePoint 2007. I know this is very akward question, but right now I am asked this very same question from client.
You cannot install SharePoint 2010 WSP on a SharePoint 2007 environment, because it references the SharePoint 2010 DLL files and the package Product Version is set to 14.0.
If you got the visual studio project, you could try and downgrade the project so that it works for 2007. More information about this can be found on this blog post.
If the WSP has any assemblies in it - no.
The assemblies developed for SharePoint 2010 will be referencing the SharePoint 2010 core assemblies, and will fail when trying to load them in SharePoint 2007.
If the WSP only has XML in it, and no server side code in assemblies, it should deploy fine, given it has no Feature code that's new in 2010 (such as Feature upgrades, or any of the newer attributes that some elements may have).
Here are some scripts that can be used to deploy the wsp to MOSS2007 to multiple site collections.