The SDK documentation has always recommended that you install plug-ins in the App Data folder:
In Windows XP, "C:\Documents and Users\username\Application
In Windows Vista, "C:\Users\username\AppData\Roaming\Adobe\Lightroom\Modules"
If you've followed this advice, you can stop reading this message. Nothing will change for you.
It has always been possible, though not documented or recommended, to install plug-ins in the folder "C:\Program Files\Adobe\Adobe Photoshop Lightroom 2\Modules\".
When we release the final of version 2.3, this path will be changing to "C:\Program Files\Adobe\Adobe Photoshop Lightroom 2.3\Modules\" and we will not be moving files from the old location to the new.
You will either need to move those files manually, or (better yet) move them to the App Data folder instead.
Mac users: We're not planning any change to the install paths on the Mac for 2.3, but this is a good opportunity for you to verify that you are installing your plug-ins in ~/Library/Application Support/Adobe/Lightroom/Modules, as recommended, and not inside the Lightroom package in the /Applications folder.