Being a reader for a number of years now, I honestly had no idea that the site was 'mirrored' accross both the .com and .co.uk domains, being an avid .co.uk user myself.
Being slightly unaware of the fact that both of these domains existed, my initial question is whether there is content that is directed exclusively at one or the other's territory (i.e. are there articles that only appear on the .co.uk or .com version of the site)? If the answer is no, then I do not see any reason to abandon the .co.uk domain for the .com one. (I guess I am also asking what is the use of a us.theregister.co.uk or uk.theregister.com sub-domain?)
After all, all major companies around the globe have to have there headquarters somewhere, and so (to me, at least) it is really a question of where your operations ultimately run from and this should determine which domain you run from. If you remain true to your current brand, then surely .co.uk is the way forward. However, one can sympathise with the decision to migrate to a .com domain, especially if this is what was originally intended.
Personally? I agree with Mark above - make a stand and adopt a us.theregister.co.uk or, maybe, force them into a us.theregister.com domain to show that the .com domain should really be non-geographic.
Anyway, kudos for this article - it says so much about El Reg's approach to journalism and the modesty with which it consults it's readership.