They have stated clearly that it will only connect via Ad-Hoc mode (as do most online PSP games), which uses Wi-Fi to connect directly to systems within a close proximity. However, it is also going to support Ad-Hoc party, which is a PS3 app that tricks the PSP into going online by using the PS3 as an Ad-Hoc connection and then making other PSPs using the application appear to be connected directly.
You can get the details from
Jordan's report on it on the news page from last month.
Also, since then Sony has filed for an Ad-Hoc Party patent in the US, so the signs that they are working to get the system in the US is there.
It should be noted that in the interviews where they mention how long it would take to get all 800+ cars by yourself, calling it impossible, they also often refer to how most people could never obtain all the cars in GT4. But as anyone on GTP knows, there are people on here that got every single GT4 car on their own.
I honestly believe the car trading is just to aid the majority of average players who won't, or can't, put the effort into obtaining every single car on their own. This makes sense, as GT PSP is supposed to be designed to draw new fans into the franchise.