Your SVN passwords in Ubuntu (12.04) are in:
~/.subversion/auth/svn.simple/
However in newer versions they are encrypted, as earlier someone mentioned. To find gnome-keyring passwords, I suggest You to use 'gkeyring' program.
To install it on Ubuntu – add repository :
sudo add-apt-repository ppa:kampka/ppa
sudo apt-get update
Install it:
sudo apt-get install gkeyring
And run as following:
gkeyring --id 15 --output=name,secret
Try different key ids to find pair matching what you are looking for. Thanks to kampka for the soft.