The main issue with AndNav2 is availability, as their site lists versions for Germany, France, Denmark, the United Kingdom, Spain, Switzerland, Italy, Austria and Ireland, but not the US. This could be an exclusion based on insufficient mapping data for the country, but in any case there's no reason that you couldn't install the British version and at least try it. The alpha release of AndNav2 is available here.
UPDATE: Nicolas Gramlich, the man behind AndNav, got in touch to let us know why there's no US version: logistics. Opening the app to running routes through North America would require a much larger server than the one they're using right now, for which the developers are trying to secure donations (you know, from you). And honestly, the app is free, so settle down, quit yer whinin', etc. [AndNav via MobilitySite]