Didn't seem to work for ePub attachments (and yes the Kindle app was installed)
Strange... I did it for just that...?
UPDATE: I was curious, so tried again. It worked for my other book readers (stanza, megareader), but apparently Nook nor Kindle set themselves up to be able to handle epub. Strange oversight, but that's more B&N and Amazon's fault (I'd say it was b/c they don't want people using their apps to read non-drm books...)