This Pidgin plugin allows you to store your passwords encrypted within KDE's wallet (by default Pidgin stored them in clear text under ~/.purple)
The plugin will move your passwords from ~/.purple/accounts.xml into the KDE wallet. It is activated via the 'Tools/KWallet/Encrypt Password' menu item. You will need to call this each time you change an account's password.
This is a port to KDE4's KWallet DBus API from the original kwallet_password.c
which can be found at: http://kainaw.com/libpurple/
p.s. If knew how to have the saving automated, and update when a password is changed, then I would've added this. However, my quick attempts failed.