Yeah, unlocking the difficulties is one of the good things about the single player mode ;) I think people saying that the game will be boring because of it's lack of career mode haven't yet seen the full scope of the game, there are so many thing to unlock, time trials, drift races, earning all golds on the driving missions, which come in two sets, and earning all the cars, will have you going on with this game for a long while xD.

@lestat

would the game be fun for someone who's never played a GT game before?

@silicon

Well, I believe it to be one of the best racing experiences on a portable console, but it depends on whether you like a more fast paced racing experience like WipeOut or Need For Speed, or you like a more simulation based racing experience.
In GT PSP you can play almost in both styles, since if you drive on the standart car physics, it will be very arcady, and if you put it on professional, you'll have a heavily sim based experience.
Other thing to consider is that the main soul of this game is to collect all the cars for GT5, since everything in this game will give you a huge amount of money for that same purpose, which, in combination to what I said before about the unlockables, will have you playing it for a long while. Heck, I completed almost all driving mission challenges with gold, but i only got 17 cars from the 800 pool, so you see how long I still have to go on the game ;)