The only way (that I know of so far) to get that setting on a iPhone 3G (which is what the OP has) is to first upgrade to 3.0 OS, jailbreak it, and then install "Show Battery Percentage". Yes I know there are other jb apps (e.g., SBsettings) that'll enable the battery percentage meter in the status bar but "Show Battery Percentage" is the only one I'm aware of so far that allows you to enable/disable the percentage meter in the same manner that it's done on a 3GS.Check under setting then general then usage. See if there is a on/off switch that says battery percentage.
There is at least one non-jb app that can display battery level as a percentage but none that I'm aware of that show it in the status bar (w/ toggle on/off display setting) like the native one in the 3GS or in a jb 3G 3.0 (either w/ or w/o toggle setting).Thank you tonepod this is the solution I was looking for. But isn't there any way to do that without jailbreaking it.