I received 2 replacements just this week. The X I got on release day started locking up and requiring a battery pull to even boot again. After a factory reset, with no additional apps installed, the issue continued. Went to store on Friday, got replacement overnighted and received on Monday. The one they sent on Monday would not connect to Google. Had 3g for everything else, but could not add a Google account. Went into store and 3 different people played for over an hour and could not get it to work. Soooo they overnighted me a replacement for the replacement, which I got on Tuesday. In both cases, they sent a X with no battery and no back. It is how they send the Refurbs, or new phones, if they don't have any refurbs yet, like now.
If you are having a replacement shipped, you'll definitely be keeping the battery, because they don't send one. You won't end up with an extra though. If they gave you a replacement in the store, I'm assuming they would keep the defective phone right then, Either way, you're not getting an extra battery "for your troubles"