Are Adobe's plugin architectures for Photoshop and Lightroom related in any way? If I have source code for a plugin, that works with PS 3.0-CS3 as well as PS Elements 6.0 can I use it with Lightroom directly? If not, what would I have to modify?
No.. lightroom plugins are written in the scripting language lua, photoshop plugins are written in C++.
As noted by kasperjj, the Lightroom plugins are written in Lua so there is not a direct way to convert something from Photoshop over to Lightroom. Additionally, as per the Adobe Lightroom Developer Center, the only features that are extendable in the current SDK are the export functionality, metadata, and web engine functionality.
As pointed out by Rob, the Lightroom SDK does not expose any interface that allows manipulations of the image files themselves. Partly this is because Lightroom is a non-destructive editor. None of the edits made in its Develop module are applied to the original image file; they are applied to the file generated when the image is exported, printed, or used in a web gallery.
That said, there are examples of export plugins that manipulate the image after Lightroom has finished applying its adjustments. In principle, it would be possible to create a host application that loaded a Photoshop plugin and applied it during Lightroom export. It might even be possible to use Photoshop itself as that host application...