The short answer is, yes, the plug-ins will need a recompile. We have extended the API in several places. Both Clive and I have tried to be pretty careful, for example, deriving new interfaces rather than changing existing ones, so a recompile isn't strictly necessary, but this obviously doesn't rule out all breaking changes.
As a result we've revved the assembly version number to force a recompile. What this means is that we can be sure that any problems reported really are problems with Reflector, and not with one of the add-ins manifesting some incompatibility with the new version.
We've already started working with the active add-in authors to help ensure that their add-ins are compatible with the new version and, once v7 has stabilised, I fully expect to see compatible add-ins being released early next year.
Hope that helps!