Redsn0w which is currently the only option to jailbreak iPhone using iOS4 for iPhone 3GS, iPhone 3G and iPod 2G has been updated to add new features which includes a Verbose Kernel boot and ability to display battery percentages.
With Verbose Kernel boot, you will be able to view the boot sequence which is similar to how Linux boots happen. Check the video below to view the Verbose Kernel boot.
In addition to that, you can also view the percentage for the battery by changing the settings under Settings->General->Usage.
Please note that the current iOS4 jailbreak is only available for the GM version. iOS4 will be released free for iPhone 3GS, iPhone 3G and iPod Touch shortly, so if you are not a developer, don’t use the redsn0w jailbreak as of yet.
More info here.