CVS and "product lines"?

[Klaus Marius Hansen]

We have two projects ("A" & "B") that share a common base framework
("Common").

We would like to be able to have a main development branch (for
introducing new functionality) and the for each new version of A and B,
be able to create a branch for a project. 

The problem is then Common. If we make a bug-fix in "Common", we would
like to have it show up in both the branch for A and the branch for B.

Does anyone have experience with a situation like this and have some
advice to give?

Regards,
        Klaus