Chris Charabaruk writes in his blog:
Do you protect your updates on Twitter? Well, don’t bother. Thanks to the magic that is Twitter Search, the whole world can see what you tweet with little trouble, whether you protect your tweets or not.
Er, no.
When and while your account is unprotected, all tweets you post go out into the search index of Twitter Search, Google, and into FriendFeed at least. When you protect your account, these don’t get deleted – what’s out is out, there is no way to take it back.
Tweets sent while your account is protected will not be indexed in twitter search, they will not be visible to Google, and AFAIK they won’t be sent to FriendFeed either.
Keep in mind that if you unprotect your account later, all your tweets can potentially be indexed. Twitter Search does not currently do this, they only index new tweets. Google doesn’t normally crawl far back. But still, keep in mind it’s possible.
Unless you have a reproducible bug – which you should then report to Twitter, please – don’t make huge claims.