KDE and Gnome already provide implementations with their own APIs, to be shared amongst their various applications. Mozilla has another, but I don't know if separate instances of Firefox and Thunderbird run by the same user can actually share the secret store between them. Every mail client and most browsers will store authentication credentials for you if you so desire. Putty and network-manager do too. Ssh provides a forwarding agent for sharing credentials across machines. We used to have to put our dialup or PPPoE credentials in plaintext in a 'secrets' file under /etc/ppp.
The problem that a new unified secrets API is proposed to solve is not the absence of a keeping place for secrets or an API for that keeping place, it is the incompatibility of the existing implementations: to date, there is no way to share your secrets among the programs you run, you have to copy them about manually. The exceptions are that you *can* share them amongst GNOME applications and, separately, amongst KDE applications.
See also 'monkeysphere', recently announced, which tries to solve a similar problem (in a completely different way).