I have a Japanese White PSP 1000 Value Pack - it's over a year and a half old. I bought it second hand so personally I've had it 6 months or so. Anywho there's no dirt or marks on it at all, so I don't get the 2 comments about people's white PSPs turning yellow. Maybe the plastic they used in later batches was slightly different (like with SNES machines)?
Well mine is yellowing where the fingers go on the back of mine.
the difference between the color inside the umd ring and on the battery cover (where I hold my psp) is quite noticable