We know, it's been there since 1.0.0. It's not going to be a "feature" ... it will not be accessible without hacking. That means it's not a feature.
Here's a tutorial for enabling it on 1.1.1, I can't be bothered to find an older one for 1.0.0, you'll just have to take my word for it.
http://forums.hardwarezone.com.sg/showthread.php?t=1916604
Internal testing builds probably have these hidden options accessible so engineers and testers can get accurate readings of stuff. That would explain how this guy has it on his phone.
I'm not sure why you think you need Winterboard or BossPrefs to enable the battery percentage indicator. That's just incorrect. You'll find a tutorial link earlier in this thread showing how to enable the battery indicator on 3.0 by hacking it.
My logic is that if you have to hack your phone to get it, then it's not a feature. That's why this shouldn't be considered a feature. It will not be available (without hacking) on 3.0. That much is apparent looking at the history of iPhone software versions. 1.0 - present, the battery percentage indicator has been a hidden "feature" that could be enabled via hacking, and it hasn't ever been an official feature which is accessible without hacking.
It's not a feature, it's a hack.