I've only been upgraded twice in all my stays as a Diamond, and only once from a standard room (at the GH Denver). I was upgraded to the Penthouse Suite at PH Chicago earlier this year, but I was already booked in a Lakeview Suite.

Based on the review threads here, I conclude that check-in upgrades are mostly property-dependent. There are certain properties that seem to consistently upgrade Diamonds. I certainly don't expect upgrades simply due to my Diamond status.