What do you like/dislike about the X? I haven't heard of any new large screen phones on the horizon - but that doesn't mean there won't be any. If it's processor power or screen quality or World Phone abilities there will always be a new phone about to be released. If you want LTE I wouldn't hold your breath unless you NEED it the day it comes out and you're OK with keeping your Eris until mid next year.
That said, what's wrong with your Eris and can you hold on to it? The longer you wait the more "badass" your replacement phone will be. But I've got a friend with an ancient G1 who's very envious of the Droid, Nexus One, and X who sit next to him...