The 1828 Small Date is the first Proof Dime struck in a close collar. The new technology allowed mint employees to create coins of uniform diameter and a more even strike.
Breen (1989) cited five Proof examples of JR-1 (1828 Small Date). Davis et al (1984) also cited five examples, the finest known to them being a raw Proof-67 sold by Kamal Ahwash in a 1979 private sale.
There are no Proofs known of the 1828 Large Date, which is as expected -- this variety was struck in 1829 using a reverse previously used on an 1829 Dime!
As of October 2010, the PCGS Population Report showed a single PR65 (graded in 1988). Two PR64's (the same coin perhaps?) were both removed from their holders at a later date.
Sources and/or recommended reading:
"Walter Breen's Encyclopedia of United States and Colonial Proof Coins 1722-1989, New Revised and Corrected Edition" by Walter Breen (1989).
"Early United States Dimes 1796-1837" by David Davis, Russell J. Logan, Allen F. Lovejoy, John W. McCloskey and William L. Subjack (1984)