Saw this Twitter status update:
  Does the #bitcoin protocol have a version number that allows to upgrade if someone leaves the system for 5 - 10 years?
  http://twitter.com/stephanwehner/statuses/16224034316361728

I think what is being asked is, if a client were not upgraded for a very long time, could it still communicate?

Either way, will the future clients be backward compatible as far as being able to access today's wallet.dat?

A lot can happen in 5 to 10 years. 

  IPv6, various vulnerabilities discovered, encrypted wallet.dat, etc.

(Incidentally, ... isn't the reason most who have left the system for 5 - 10 years due to being incarcerated?)

How would I merge the keys from an old wallet.dat into current.  If I can launch the client, I can just transfer to address from a new wallet, but is it possible to add those keys into another wallet?

Just to bookend this thread:
  Pull #220, Wallet and key import and export
  - https://github.com/bitcoin/bitcoin/pull/220