I'm not a Debian/Kubuntu man, but I think that means to
remove Soprano 0.9.0 (stable) and replace it with
Soprano 1.99 (RC2), which might not be stable.

what you call not stable, is used by every distributions and set as a minimal requirement for KDE 4.0.0 .
For informations, soprano 2.0.0 was released yesterday.

cheers,

Fathi