For my iPhone 3G this came, out of the box, right after the update to 3.0. I could clearly see the 100% next to my battery icon.

But yes, I did have my iPhone jailbroken before I upgraded.

Now, appearantly, this feature is only available for iPhone 3GS, and not iPhone 3G and iPhone 2G, though, it is possible using e.g. Bossprefs. Lame from Apple !

Anyhow, after some Googling, I came to the conclusion:

• iPhones that used to be jailbroken will have the % displayed next to the battery icon.
• Regular iPhone 2G and 3G will not have the switch in "Settings > General > Usage", whereas 3GS will.

I found a cool workaround right here, although it requires you to jailbreak your iPhone.

Don’t forget to install “openssh”  and “wget” or “lynx” (makes it a lot easier) using the Cydia package manager.

I’ve hosted the plist file right here, just in case it would go offline.

Basicly, you have to add these 2 lines to the .plist:

   <key>gas-gauge-battery</key>
   <true/>

This works with the current iPhone version (3.0).

I’ve copy pasted the howto, just in case. All credits go to the original poster.

1. First you need to Jailbreak your iPhone, use PwnageTool or Redsnow check the dev team blog. -> and install openSSH, wget, lynx, and restart your iPhone to be sure.
2. Download the M68AP.plist file from the link: http://www.4shared.com/file/113188452/9434236e/M68AP.html or from the link I've posted above. This is optional, see #5
3. SSH into your iPhone. (Or use iPhoneBrowser 'look it up in google its right there!) -> if this is first time you SSH to your iPhone it might take hell of a long time and even time out a few times. Due to the keys being generated. Use PuTTY or Terminal to ssh; command: ssh -l root <iPhone IP>. The default password is "alpine" (don't forget to change that with "passwd").
4. Go to /System/Library/CoreServices/Springboard.app/
5. Replace the M68AP.plist in the directory with the one you downloaded -> you might aswell "mv M68AP.plist M68AP.plist.backup && wget https://static.yeri.be/2009/07/M68AP.plist
6. Restart you iphone.
7. Now go to the application Settings -> General -> Usage and there will be a Battery Percentage toggle and you can use it to enable battery percentage next to battery icon in top-right corner.

And that works !