As this is in a comment on the last post, I thought I’d mention this:
Lightroom does not use Camera Raw as a plugin. The two applications share the same codebase and are generally released at the same time.
In the About Lightroom box for 2.1, it states Camera Raw 4.6. This merely indicates that the 2 are compatible for use with Edit in Photoshop.
Where people are getting worried is with the fact that they don’t know that Camera Raw 4.6 and 5.1 are actually compatible.
Camera Raw 4.6 is the final Camera Raw release for Photoshop CS3. Camera Raw 5.1 updates Camera Raw 5.0 for Photoshop CS4 to be compatible with 4.6. This means that, for now, users of Lightroom 2.1, Photoshop CS3 and Photoshop CS4 are in sync for Raw processing.
When Lightroom updates to 2.2 and Camera Raw to 2.2 (I suspect this will happen for historical reasons, if nothing else), then Photoshop CS3 users will be left behind. Of course the DNG convertor will be updated and you can still access changes that way.
So don’t worry about updating Lightroom 2.1 to Camera Raw 5.1. It already is.