Sharding was introduced for FSFS repositories in Subversion 1.5, but only now I moved to a sharded repository (at time of this writing: Subversion 1.6.1). This meant - as usual - a dump / create / load dance. Followed by the pack operation:

svnadmin dump c:\subversion\sharpdevelop > c:\sd.dump
svnadmin create c:\subversion\sharpdevelop
svnadmin load c:\subversion\sharpdevelop < c:\sd.dump
svnadmin pack c:\subversion\sharpdevelop

Without svnadmin pack, you only get sharding. Executing svnadmin pack puts all the shards together into a single file. This makes certain operations such as svnadmin hotcopy way faster.