Nick,
I've been experimenting. With the settings shown in this screen grab, data that I add in PM appears in LR when the images are imported and will update when the metadata is synched in Lightroom too (Metadata/Read Metadata from File).
I do not recommend embedding IPTC or XMP inside your RAW files if you're working with Adobe's applications. If you do this then you'll encounter a bug in their applications where they won't update the embedded data when it changes and only update the sidecar file and moreover Adobe's apps will prefer the stagnant embedded data over the newer sidecar data.
You may not be having problems yet, but you
will encounter them at some point.
You need to use a XMP sidecar only workflow and get LR to update its internal database metadata when the sidecar file changes externally. Please read the Lightroom documentation and see how one instructs the application to resync its metadata with the metadata stored on the file system.
EDIT: I fired up Lightroom 2. While I don't usually use LR myself, I found out how you get LR to realize the changes. Right-click on a selected photo and choose Metadata->Read Metadata from File from the contextual menu that appears. Click on the Read button when the warning dialog appears. You can check the "Don't show again" checkbox if you don't want to have to confirm the operation each time. You can also use the Metadata menu on the main menubar on the selected photos by choosing "Read metadata from Files".
For best interoperability I suggest you set LR's Catalog Metadata settings to 'Automatically write changes into XMP' so that when you make changes in LR, other applications like Photo Mechanic will see them as well.
Please use an XMP sidecar only workflow with Adobe's applications. You'll be much happier in the long run.
HTH,
-Kirk