however i will say this. the hole for the mute button is very deep and i find it quite difficult to actually move the button. if you have really long nails then it will probably be ok, but i don't and it's quite difficult to get to it.
i also find that the volume buttons don't sit quite perfect. many times whilst taking a photo i have accidentally taken a photo but simply touching the edge of the bumper.
There are old style bumpers floating around on amazon and other non-apple sites that only fit the AT&T iPhone 4. I believe they have a black packaging background.
Mainly, if you buy one that comes in white packaging, it's the new one.
The new ones are definitely compatible, had one on mine for awhile til I switched to another case. I will echo others comments though, make sure it's not an old one as the mute/volume button area is different.
Yes new ones work work. The way to tell is the copyright date on the back. I believe if it's later than 2011 (or 2010) then it's compatible. They only changed it once, the placement is identical when the new VZ iPhone was released. However, the original bumper still work but it partly covers the mute button, but not entirely. You still have access to it.