Roku CR app is SD only, the AppleTV version supports 1080p or the max resolution of the series you're watching. Boxee supported HD, as well as PS3, never was sure why Roku never got HD support but its why I never bothered with it.
Crunchyroll claims that you get 1080p on a Roku (it needs to be one that supports 1080p) if you are a premium member. Otherwise, you get SD.