I finally got a PS3 recently. I've been trying to catch up on everything I missed, so I bought inFamous a couple of days ago, but now I've discovered that it's free from the PlayStation store. So I was thinking I'd return the game and take advantage of the free offer. But if I do that, will I have to start over, or will inFamous be able to recognize its saved games and achievements from the disc?
You might not like this answer, but if you didn't have a PSN account in April, you won't qualify for the freebies. If you have only just purchased your PS3, then this will be the case, and you should just keep the disc.
On the other hand, if you do qualify for the freebies, then yes--the games should share the data. I base this on the example of Dead Space: Extraction. I had the game on PSN before I purchased Dead Space 2 Collectors Edition, which came with a copy of the game on the disc. I installed Extraction from the disc, and then launched it from the disc--it shared the data from my previous PSN version of the game.
Of course, it could just be a special case with Extraction, but that would be quite unexpected.