There is an Android equivalent to Microsoft's CTRL+Z?
The back button serves as Undo or Cancel (or Back) in many apps, but won't help with a typo in a text box, it's more likely to lose everything you've typed in there than undo the last bit of typing.

I don't believe that there's any standard, central Undo function in Android at the moment. As very few devices have a hardware keyboard, and many devices are starting to drop even the four hard buttons, I'm not sure how Undo could easily be invoked/shown to the user. Ctrl/Alt etc key combos obviously aren't practical on a soft keyboard on a 4 inch touchscreen, and I'm not sure how else it would be done except with a dedicated undo button on the phone or the soft-keyboard.