I've confirmed that Calibre doesn't store its metadata info in the mobi itself until you do one of three things:
1) Save to Disk - it records the changes that you've made to the metadata, but retains the Amazon records that were unaffected by your changes (ASIN-113, cdetype-501 are still there if they existed to begin with). You can configure Calibre to not save metadata changes when using the Save to disk option (Preferences->Add/Save->Saving books tab).
2) Send to Device - it records the changes that you've made to the metadata, but retains the Amazon records that were unaffected by your changes (ASIN-113, cdetype-501 are still there if they existed to begin with). You can configure how Calibre synchronizes metadata when using the Send to Device option (Preferences->Add/Save->Sending to device tab). I'm guessing the "Automatic management" option doesn't write metadata to the exth header, but rather synchronizes metadata between library data and the metadata.calibre file (on the Kindle) on every device connect.
3) Mobi 2 Mobi conversion - This overwrites
all exth records so only Calibre's metadata is written to the mobi file (ASIN-113, cdetype-501 are erased if they existed to begin with).
What's it all mean??? My stripped Amazon books retain their ability to synchronize across devices (K4PC and K2). Furthest Read point, Notes & Highlights, and Bookmarks are kept in sync (at east all the books I've tried thus far). I've even been able to synchronize books that I didn't purchase from Amazon by adding a valid ASIN (in exth-type 113) and cdetype 'EBOK' (in exth-type 501) with the mobi2mobi perl script. In this case, the Furthest Read point is synced, but not the Notes & Highlights or Bookmarks.
If you have some deDRMed Amazon books that do not synchronize, I suggest using the mobi2mobi script to verify that the ASIN record (113) and the cdetype record (501) do indeed exist and that they contain a valid ASIN and 'EBOK', respectively.
The Social Networking issue (showing the correct cover and book title) appears to be a little more complicated.
