I don't use versioned dependencies in the backports. However, generally
the higher version number would be given preference (based on my also
limited understanding of apt, as it sometimes still surprises me).
Possibly you have something in your /etc/apt/preferences, or maybe you
even used tagging when installing r-cran-boot?
Just thoughts... Anyway if it is not /etc/apt/preferences causing this
behavior, you can probably still just
apt-get install -t lenny-cran r-cran-boot.
The problem was due to my limited understanding of APT after all. As you
had correctly suspected, the culprit was the configuration in my
/etc/apt/preferences file.
I had given lenny-cran a lower priority than lenny.
Thanks again,