While, at time of writing, the Gran Turismo 5 demo is unavailable in the US or PAL territories, it is currently available from the Japanese PlayStation Store. And is 100% "import" friendly.
If you've figured out how to create a Japanese account (here's a guide if you haven't), you can get the demo right now, and do so in the knowledge that the whole thing is in English (like the Prologue demo, it detects your system's language and adjusts accordingly).
Strangely, though, the demo is only a bit over 200mb in size, and is...well, as you'd expect from 200mb in 2009, it's pretty scarce. Actually, it's very scarce, as after only an half an hour I was already bored with it. Unless you're genuinely expecting to compete in the Time Trial Academy (the winners of which will get a shot at racing real race cars), there's not much here to keep you busy.
Maybe they should have thrown another track in there, add another car or two...