I recently downloaded and installed several trials of CS6 including Illustrator, Photoshop, and InDesign- these installed fine. I also attempted to download and install Premiere Pro. The software seems to have downloaded however, when I attempt to open the .dmg the file opens and once complete a warning appears saying "the following disk images couldn't be opened: PremierePro_6_LS7.dmg with reason being 'invalid checksum.'" I have already attempted re-downloading the software. I am currently running OSX 10.7.3.
Is there any messages in the Adobe Download Assistant that it is still extracting or downloading? This often occurs when the download is not yet complete.
Alternately you can follow the steps listed at http://forums.adobe.com/thread/981369 to initiate a direct download.
Was it just writing over the previously downloaded install files? That might be what caused the problem. You could try disabling the checksum feature in the preferences under Disk utility. But I would recommnend deleting the install files and trying to download either through the Adobe Download Assistant or the direct download option provided earlier.