Can iPad show percentage when charging by USB, and does iPad 2 charge slowly too? I found that my iPad might be half charged after a few hours by USB from PC... too bad I can't find the wall adapter...  but when I turn it on, it says "Not charging" without saying the Percentage it already has charged up.  Can it show somehow?  Maybe by tapping the "Not charging" message but I don't really need to see it so many times. (but tapping doesn't work)

Does the iPad 2 also have the same issue of slow charging too?
The reason you get the Not Charging message is because most machines don't send enough power through USB to charge the iPad. The iPads battery is more like a laptop battery than a phone battery. Newer Macs will send enough power over some of their USB ports. 

As for the percentage, If you turn on "Battery Percentage" I think under Settings, General, Usage. This should show it up in the top right. 

Due to the iPad 1 and 2 sharing many other similarities, the iPad 2 should also display the 'not charging' message. Although, a new iPad 2 will come with a Wall Charger as well. You can purchase off-brand chargers on amazon or ebay, extra wall chargers at the apple store, and some third party companies make chargers like Griffin