I don't know if you guys know this already, but you can enable this on 2G's and 3G's that don't show the battery percentage slider very easily if you have jailbroken your iPhone/iTouch.
SSH into your iphone
Go to /System/Library/CoreServices/Springboard.app/
Download the file M68AP.plist
Open the file using a text editor or a plist editor
Add a Boolean value under the capabilities dictionary
Boolean Key = gas-gauge-battery / Boolean Value = true
Save and upload the file to the same directory that it was originally in
Go onto the iPhone and respring /restart
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.
(from funkyspacemonkey)
Works fine on my 2G iPhone that didn't show the slider before.