Considering the fact that you've said you DO have exceptional SPaG, I will definitely offer to be your beta. :) I like to think I have those same skills, especially since the current career I am drooling over (that is, something at which I could actually see myself being able to make a living) is some form of editor. I beta for quite a few people, so you can ask around. I'm from the US, too, so that meets one of your requirements. :)

So yes, if you feel like you can trust me as a beta, let me know & I'll give you my email addy. :)