OP:
I'd second Kimberli's comments and add an additional perspective - while one may consider going outside the US for residency and fellowships (especially for competitive residencies), you have to realize that just as US residencies prefer "their own", so to do foreign programs as well.
In addition, many foreign programs require much longer training. I'll take ENT for example (as it is what I'm interested in) - In Australia, to become a fellow of the Royal Australian College of Surgeons as an ENT, one must first do a rotating internship (1 year), basic surgical training (2 years), sit the basic surgical training exam, and then apply for ENT training (which is 4 years) - mind you that ~30-40% progress to advanced training after passing the BST exam and currently there is a 3-4 year wait list to get into ENT. Thus one is looking at 1+2+3+4 (Read 10+) years to be an ENT after completion of medical school.... And then must deal with transferring training back to the US - as a non BC/BE surgeon.
The training here in the US is definitely more accelerated. If you desire to practice in the USA, I find it quite difficult to justify training elsewhere. Similarly, if one wishes to practice in the UK/OZ or elsewhere, it is best to do postgraduate training there.
That being said, I do think that advanced (ie fellowship) training abroad has its advantages - indeed once at this level it is best to train under someone who possesses the skills that you seek to achieve. Of course, after having completed a US residency, BC/BE will not be an issue, regardless of where the fellowship was undertaken (although I could be mistaken).
Hope this helps.
With kind regards,
Airborne