You have to delete the "blkindex.dat" file whenever you change the wallet.dat so it can re-search for the block index. 

can a wallet.dat file be used on any bitcoin prog on anycomputer if put in the right folder(while the orginal owners wallet is moved temp to a new safe location)

what about having 2 wallets? do you have to back it up all over again every time the amount changes?, what if you spend some btc and then put in your old wallet file

ya no one is giving out 2BTC prolly now, but how about .05BTC as a test 1DkABmGmWeaF5DJTw2BCqzo6Cx3u2fyNz