After installing UbuntuOne for KDE via KPackageManager I have to inform my authentication password twice on every boot: one for KWallet and other for a "keyring", in this order. Is it possible to authenticate only once, in KWallet?
Keyring is a GNOME thing. KWallet is a KDE thing. The UbuntuOne client is written with GNOME in mind, so it uses the Keyring. There is no KDE version of the client.
What you could do is install seahorse to manage the GNOME keyring, then set the login keyring password for GNOME to blank, so then it won't bother asking on login because it wouldn't be storing the authentication information encrypted at all... which obviously has its own security implications.
Currently Ubuntu One uses GNOME Keyring to store credentials on Ubuntu.
In fact, there is code in the client to use python-keyring (we use it on the windows port), which supports KWallet, so it's not outside the realm of possibility that someday we may support KWallet automatically on KDE.
However, there are no resources allocated to it at this time.
However^2 the client is open source ;-)