You’ve said “The iPad with 3G, which is locked to a mobile carrier…”, but it’s not locked. It’s unlocked. Mr Jobs says so in the keynote at 1:14:33. I’ve uploaded an excerpt of it here for you: http://www.youtube.com/watch?v=vakJfz1X7aoI don’t see any reason at all that all iPad models won’t be supported for warranty issues globally.
Now that more news has come out about the iPad it looks like the iPad is locked to one carrier in Japan, but can accept SIM cards from other countries. Softbank has also stated that it will not support iPads from foreign countries. This presents repair problems for anyone who brings in their foreign iPad to a Japanese Apple Store. I assume iPad repair policy will look like iPhone repair policy: repairs can only be done in country of purchase.