As of v0.7.44, Calibre automatically adds this functionality to .mobi files.
MOBI Output: Use the book uuid as the ASIN field and set cdetype to EBOK to allow Amazon furthest read tracking to work with calibre generated MOBI files.
Closes tickets: 8721
Note that you cannot currently customize what ASIN is used (through the Calibre user interface). Mobi2Mobi is no longer required for this functionality, unless you want to set the ASIN value yourself, perhaps to the Kindle or paperback ASIN.
I requested this feature of Kovid maybe a week ago (daffy4u had originally requested it but I think that request was closed due to a simple misunderstanding) and he added it right in just a few days ago.
I verified it with several books that I converted from .pdf to .mobi, and they are syncing perfectly between my Kindle and Kindle for iPhone (both directions, no problems).
Thanks so much Kovid!!