I ran into some issues with 1.6.2 encoding a map to a JSON structure because it had int/int64 keys. Sure, JSON prescribes string-only keys, but I incorrectly made the reasonable assumption that Go would coerce these to strings. It turns out that this change was made in the very recent past and that, as of the last release (1.7.1) (and probably earlier ones) this should no longer be a problem.
Unfortunately, AppEngine still runs on 1.6.2, for now. So, if you had the same problem, you will have to go the conventional route and translate these keys to strings, yourself, prior to marshaling.