Glad I was of help.
I do have some technical knowledge on redirections, however, as we are dealing with subdomains here, I'm not sure cookies will work. Remember that x.domain.com is a different domain than y.domain.com, thus making a cookie created by x.domain.com useless on y.domain.com.
I've checked a couple of sites that do this kind of redirection, and can't hardly found an example of it using cookies, I find lots of them using subfolders: domain.com/au/, etc. as the cookie is valid for all subfolders.
How about forgeting about a "Global" cookie, and just using one for the particular subdomain (if you still want to go with the subdomain route), here's how it will work:
domain.com -> redirects to the "best match" (US go to us version, AU go to au version, others go to whatever version you consider the "default").
Then, in the subdomain, you implement the lightbox pop-up (the less intrusive one you can come up with) and save their response, so if the user accesses the next day to au.domain.com they won't be prompted again to change the location, BUT if they access domain.com (a US visitor) he/she will be redirected to the US version and get the lightbox again.
You end up "basically" with the same results, however, it could be a little annoying for some users, at least I know I would be annoyed if that happened to me.
Give it a day and think if subfolders aren't better in your case, that should solve all problems, and implementation will as easy as 1,2,3 (I am capable of helping you with that approach). You won't be using cookies, but session variables (although cookies will allow you to remember the user choice for any time-frame you want).
Oops, forgot to mention, 302 redirects 