Hi Matteo,
I think it's generally accepted that IP redirection in this scenario isn't considered cloaking by the search engines.
However I have previously seen sites seemingly penalised (possibly because they're tripping some sort of cloaking filter) but in my experience this more commonly happens when different content is shown on the same URL based on IP detection, rather than redirecting to an alternative page.
Cloaking issues aside, you rightly point out that the redirect can cause indexation problems (as the bots may only see your US content) - as such this isn't a solution which I'm keen to recommend.
If you really need to implement something like this, you could try doing what cheapflights.com do. If you visit cheapflights.com from a UK IP they redirect you to this international choice page -
http://www.cheapflights.com/workers/profile-select.aspx?sref=CFUK&redirect=GeoIP&geoip=GB&cfref=CFUS&spt=Home&rp=/:
However, my preference would be to either -
a) Don't do any IP redirection, but upweight the links to the international sites so that there is a clear route for users to find the right version of the site.
b) Use IP detection to show a JavaScript overlay suggesting UK users might want to visit the UK site - but don't actually redirect them.
I hope this helps 
Hannah