I think this limitation may be serious enough to undermine Bitcoin in the long-term. Somebody on these forums has suggested that isolated groups (like a Mars colony) may simply start their own block-chain for local use. Not sure how exchanges between networks would work. You would probably need to use a trusted courier to move a wallet.dat (for each network) within "range" of the main network periodically.
depends on how exactly the new block chain formed. i.e. forked from the existing chain via a rule change or started completely on its own.
if the former, it could get messy.
with the latter, they would function as two independent currencies and exchanges would work the same as currency exchanges do now, bitcoin or other, though pegging the currencies to each other would likely be desirable.