I don't remember that all of the available cars in GT mode can be locked in Arcade mode in previous GT games. But this time in GT5 we will have at least 830 cars in arcade mode, because GT PSP has 830 cars and we can transfer anyone of them into GT5.Have you never played a Gran Turismo game before?
There is always 1 garage. You keep the cars you buy and modify in the GT mode in that garage.
In GT mode you can only drive the cars in your garage.
In Arcade mode, however, you can access the cars from your garage (for single-races, time-trials, split-screen, etc), but you also get a selection of unmodified cars that you can drive. This is not a garage.
Normally you would unlock these cars via progression through the races in Arcade mode. In previous titles you'd unlock tracks, also.
If you transfer from GTPSP you will simply unlock the cars that would require a bit of grinding to unlock in arcade mode.
However, where is the official statement about "GTPSP cars being transferred for arcade mode only"? At least I only see this official information, which I have posted this month. It doesn't mention anything about "arcade mode only".
http://www.gran-turismo.com/jp/news/d9677.html