I believe each invidual output will be returned as JSON (i.e.

pulumi stack output myobject

), but we don't currently have a way to return the full set of stack outputs as JSON. This is related to https://github.com/pulumi/pulumi/issues/496, but adding JSON support for just

pulumi stack output

is probably high enough value that we could do it before biting off the full feature of JSON support for all CLI verbs. Feel free to open a new issue on this.