I'll assume that by "npm addon" you're simply referring to plain old npm packages (native or js). (If not, ignore my comment.)
As far as I know there are currently no big API changes happening in v8, plus the abstraction layer for native node packages (nan, Native Abstracts for Node.js) has been, afaik, quite excellent at shielding native package developers from a lot of potential API breakage -- so generally it can be expected that there will be either no or very minor implications for native packages -- and no implications at all for non-native packages since those work purely with the node API which is not tied to the v8 API.
We'll know more once the v8 project's API changes doc will be updated with 5.1 info, and node core has created its vee-eight-5.1 branch.
EDIT: added clarification about non-native packages; clarity
