Why does Armory rescan the block chain every time it opens?
We just release version 0.90-beta, which finally solves this issue.  Armory does not need to rescan the blockchain anymore unless you import new private keys, or Armory experiences an unclean shutdown.  

With the exception of the first-time startup (to build the new databases), Armory should get into online mode within 10 seconds of Bitcoin-Qt/bitcoind finishing synchronization.