Both my husband and I purchased and put on cases for our new Motorola Droid phones. His a hard case, mine a gel/rubber one. If you have these I STRONGLY advise ditching them. He pushed his screen up twice to use the regular keyboard and the back part of the casing would scratch the underside of the screen. He promptly took the back part off and tossed it, keeping on only the front surrounding screen piece. The underside of the front screen, as Droid owners know, has the fine network of wires and stuff in it - almost looks like a solar panel.. That hard case would have damaged his phone after repeated use.
My case, while softer, was no different. While mine did not scratch the underside it did rub, unbeknownst to me. I started having problems about 4 days after getting the phone. My menus would lag when I flicked them up, my connections to Market, Internet and other apps would hang, my screen would go black and the lights for the menu and back buttons on the bottom would be off and not come on and now I am unable to access my Yahoo mail through the e-mail app (though we have determined this is likely due to something else going on). Earlier in the day before this started to occur I had taken the front surrounding screen piece off to replace my screen protector because it was peeling on one side - as soon as I did that and replaced the front piece I had NOTHING but the above issues all day.
So later that evening I took the entire cover system off the phone, turned the phone off and waited awhile. I turned the Droid back on and so far after a day now it seems to have gone back to normal. Though I still cannot get my Yahoo mail to work through the e-mail app (listed in another Forum topic, so evidently I am not the only one).
Suffice to say: If it did not come built into or put on the phone by the manufacturer, DON'T put it on... (I swear I have heard this same phrase before....)