The bad news is that, at launch at least, 3G file downloading is limited to 20MB or under. You'll therefore be downloading Minis or (real talk) system updates, but not full retail games while you're away from wi-fi. The representative specified that this was the case at launch, and the limits could change in the future.
The rep also addressed the odd proprietary memory card. As of now, there are currently no plans to use the format in any other devices. It sounds like Sony's not trying to push a new media format like it did the Blu-Ray -- this is really about making you buy one more memory card.