At the moment Android 2.2 has a pretty incomprehensible icon of a battery that shows remaining strength.
Is there a way to switch that to a numeric display (like is available on the iPhone)?
You can try Battery Notif free from Android Market.
Bardcode Scanner compatible store link
if you use cyanogenmod, there's an option to show numeric battery percentage, and numeric signal strength in dbm on the status bar
Battery indicator is a free app that does this: https://www.appbrain.com/app/com.darshancomputing.BatteryIndicator
Unfortunately, it doesn't replace the stock icon, it just adds another that displays the numbers. I'm not aware of any way to replace or change the stock icon, even with root access.
Press menu > Settings > About phone (scroll Down) > Status. There's an item called Battery level, which shows the currrent battery level in %. Also the part Battery use might be interesting. You'll find a list with Items that use the most battery there.
Settings > Display > More settings > Display battery as percentage.