On Windows Vista or later, the whole %appdata%\Bitcoin directory can be replaced with a symbolic directory link:
(0) Make sure the bitcoin client is not running.
(1) Move the %appdata%\Bitcoin directory to its target location (e.g. d:\bitcoinData)
(2) Run the command prompt as root and issue the command mklink /d "c:\users\\appdata\roaming\bitcoin" "d:\bitcoinData"
done!
This allows you to keep bitcoin client data anywhere, e.g. on a secure local fileserver.
Great, thank you! I'm surprised there isn't a setting to specify a destination location for blockchain data, etc. With the size exceeding 25GB already, it's probably well overdue.