<p>I've been thinking about the number of projects we have in-house that are still being developed using visual studio 6 and how best to migrate them forward onto visual studio 2008. The projects range in flavours of C/C++ and VB.</p>

<p>Is it better to let VS2008 convert the work-spaces into solutions, fix any compile errors and be on your merry way? Or, is it better to start with a clean solution and migrate code across project by project discarding dead code along the way?</p>

The Microsoft p&p team has recommended some
strategies
that answers this. Basically they recommend something like the project by project approach you mention. Of course, they're assuming a neatly architected application that has no nasty, dark corners from which late nights of coding and copious amounts of coffee spring from.
It doesn't hurt to let VS2008 convert the project for you and see how much effort is required to fix the errors.