You need to make develop changes by double-clicking your smart object in Photoshop and adjusting it in ACR. There is no link between your smart object and you master raw file in Lightroom.
The xmp's are for interchnaging information between separate stand-alone apps and/or workflows (e.g. open in Bridge, open file in Phoshop via ACR), but not when Lightroom hands over the data directly to Photoshop.
you can do the same edits in ACR that you do in LR.
as long as the RAW engine of ACR and LR is the same.
so for sull support of PV2012 you have to use photoshop CS6.
but .... with editing in photoshop you end up with a non RAW file anyway.
so why do you need a link to LR? just edit the image in ACR when you have it in photoshop.
i for example don´t want that PS changes the original file when i open a DNG in photoshop and make changes in ACR.
if ACR would write back these changes the image would look different in LR. nothing i want for my workflow.
Taking a little distance and looking at it from that angle, I can understand why it works the way it does now.
Still, I feel there are some missed opportunities here.
Compared to working between indesign and photoshop, when I have an indesign layout and I later edit the placed photoshop document, Indesign gives me a warning that my version is changed and I can update it to reflect the latest state.
A similar thing could be done here inside photoshop "update to latest Lightroom edits" or something similar which could be totally optional for the end user to use or not.
My confusion was mostly caused by the XMP files, it stuck in my head that these were external files holding all knowledge and no matter which Adobe program you used, it would work with those same xmp files... but I understood wrong apparently.
I still have a lot to learn, thanks for all the answers, it is very helpfull.
if you have edited your image in LR and want the new version in photoshop you can rightklick the smart object layer and select "replace content".
but keep in mind that when you open for example an DNG file in PS (via open as smart object) and change the brigthness these changes will be written into the DNG file.
thought you will not see the changes in LR until you read the new metadata from the image file.
when you send an image from LR into PS as smart object (edit in -> edit as smart object in photoshop) and then you make changes with ACR, the changes will not be written into the DNG file.