In
https://bitcointalk.org/index.php?topic=114315.20, Runeks proposed changing the message "[...] previous backups of the unencrypted wallet file will become useless as soon as you start using the new, encrypted wallet."
The concern raised in that thread is that users may misinterpret it to mean that no one would be able to use the previous backups. I thought "That would be great! How did they do that?" Alas, it isn't true :-(.
Moving the bitcoins from the current addresses to one or more new ones was proposed. That seemed like a good idea, but then all the associations we've created (Address XYZ is for payments from ...) would be lost.
The fact is, backing up an unencrypted wallet is foolish unless you already are sure that you can protect it from prying eyes. The backup feature in the client should give a warning to that effect if it doesn't already. I already encrypted, so now I can't test.
In any case "[...] previous backups of the unencrypted wallet file will become useless as soon as you start using the new, encrypted wallet" is simply not true and should be removed ASAP. A fresh install of the client can be given one of those backups and it will work just fine, though some of the coins, if received or returned as change in the new encrypted wallet will not be accessible. Still no "useless".
The sentence should say what it really means, which is "Your encrypted wallet will use addresses that any backups of your wallet (when it was unencrypted) don't have and can't get. Any coins in these new addresses cannot be spent from the backup wallet." The addition of an option to move all the coins to new addresses unavailable from the backups would be nice, but not necessary to solve this issue.
