available here. Even those who took advantage of Apple's free case offer are eligible for the $15. In fact, it also goes the reverse direction, as well: class members can continue to ask for a free bumper case as per http://support.apple.com/kb/HT4389.
Apple's emails to iPhone 4 owners says:
"Apple will provide a $15 cash payment to Class Members who send in a valid claim form. The claim form will require you to declare that you: (a) experienced antenna or reception issues with your iPhone 4; (b) were unable to return your iPhone 4 without incurring any costs; (c) were unwilling to use a case or free bumper for your iPhone 4; and (d) completed certain troubleshooting steps or are unable to complete the troubleshooting steps because you no longer own your iPhone 4."
If you want OUT of the settlement so you can sue on your own, you must postmark your request to exclude yourself by June 15, 2012.
"Antennagate" refers to the famous iPhone 4 issue whereupon holding the iPhone 4 a certain way, bridging the gap between the antennas on the lower left-hand side of the case, would cause signal degradation or even dropped calls.
Then Apple CEO Steve Jobs solved the issue by telling an end user "Just avoid holding it in that way." Seriously, Apple eventually held a press conference where they showed similar issues on competing phones, and Apple eventually offered a free case to those requesting one, as with a case the issue was not reproducible.