Recent appearances:
PCGS MS-67.  Ex - Heritage Numismatic Auctions, Inc.'s "Long Beach Signature Sale", May 31-June 2, 2001, Lot 6664, illustrated, sold for $6,555.00  From the holdings of Andy Hansen

PCGS MS-66.  Ex - Heritage Numismatic Auctions, Inc.'s "Long Beach Signature Sale", May 31-June 2, 2001, Lot 6663, not illustrated, sold for $805.00  From the Fred Firestone Collection

PCGS MS-66. Ex - Ira & Larry Goldberg Coin & Collectibles' "The Fairchild Family Trust Collection Sale", May 28-30, 2001, Lot 1044, illustrated, sold for $834.00

PCGS MS-66.  Ex - Bowers & Merena Galleries' "The Rarities Sale", January 3, 2001, Lot 402, not illustrated, sold for $862.50

NGC MS-64 Prooflike.  Ex- Heritage Numismatic Auctions, Inc.'s "October 2000 Long Beach Sale" October 5-7, 2000, Lot 8488, illustrated, sold for $373.75

Notes:
The 1921-D Silver Dollar is extremely common in all grades up to MS-64 and is readily available even in Gem condition.  In MS-66, the date is scarce; anything better is extremely rare.  Prooflike examples are very rare and Deep Mirror Prooflikes are virtually non-existent.  The finest examples certified by PCGS are 5 MS-67's, 3 MS-65 Prooflikes, and a single MS-64 Deep Mirror Prooflike.

No Proofs are known of this date, which is surprising, considering how accommodating the Mint was to Farran Zerbe when they struck Proofs for him of the P- and S-Mint 1921 Dollars.  Since most of the evidence concerning those coins is hearsay, it is entirely possible that some Proof 1921-D Dollars may have been struck, so keep an eye open for one!

Sources and/or recommended reading:
"The PCGS Population Report, April 2003" by The Professional Coin Grading Service