[R] Warnings using MASS Library

[Williams, Brian]

I am running R version 1.7.1.  In the process of checking the results from
some code against the results obtained from version 1.5.1, I encountered a
set of warning messages upon exiting from the session:

Save workspace image? [y/n/c]: y
Warning messages:
1:  namespaces may not be available when loading
2:  names in persistent strings are currently ignored

This happened only after loading the MASS library and running glm.nb.

I have read the postings of other users who have encountered these warnings.
The solutions suggested did not solve the problem for me (e.g. running
update.packages(), not surprising since I'm running the latest version of
MASS [7.1-8]).

Is there anything I can do that will cause these messages to disappear?  Can
I safely ignore them?

Thanks,
Brian