wallet.dat

[romerun]

I did md5 on my wallet.dat a while ago, it is different from the current. I thought wallet.dat is not supposed to change, coz I hear that it only contains private key. What happen if my wallet is corrupted, and I replace it with a backup ?

[deepceleron]

Your wallet always changes, it contains information about any transactions you have sent or received and the current state of the Bitcoin network (the last block seen when it was running).

Your backup, provided you made it when Bitcoin was completely closed, will be good for at least 100 new addresses you create or 100 transactions you send, however it won't be good forever if you continuously use Bitcoin. There are 100 reserve pool addresses in the wallet that are for future use - one you have exhausted these and Bitcoin has to make new addresses that aren't in the backup, you can lose money if your backup is out of date.

[DBG]

deepceleron summed it up extremely well.  Unless this wallet.dat file is an offline file that hasn't been touched by any means then the MD5 should and would have changed.