I'm having the same problem. I've had it on the Droid x and now on the Thunderbolt. The only similarities I can think of is that they are both rooted. The only workaround I found was to "unmount" the sdcard in settings to install from the google market or have the phone connected to a pc in usb storage mode. The amazon market actually downloads the apk file to a cache located on the sdcard and then installs it from there. For some reason, some apps that use the sdcard wont install and some will. I've had problems with angry birds (all three editions), runik, pocket legends, and a couple of others. The workaround I found for the amazon market was to let it download, get the error message that it couldnt install, use root explorer to locate the apk file on the sdcard, copy it over to the actual phone system, unmount the sdcard, and then install it. then delete it from the system. I posted a thread about this last week but got no responce. I don't know why this is happening. Hope this helps.