if it will be free, it will be ad supported in some sort of way. i assume this is where the premium service fits in. if you pay for the service, it might be ad free, but if you actually do pay for premium level, i would guess it will be more like xbl gold. you pay to play and you can download movie trailers(which are definitely ads) if you want to, but you are not required to download or view them ever.
my guess is that not only will the non-premium user will be forced to view these ads while their game is loading, but they will also be required to store the ads on the local ps3 hard drive. you can bet those ads will be conveniently downloaded in the background while you play a game offline(connected) or if you leave the console unattended(connected). if you never do either of those, you will be constantly watching the same ads. similar to free video clips or free software or anything else that is free, it is not free, someone is paying for the service in exchange for access to your 30-60 seconds of load time.
source instead of speculation?