I figured this deserved its own thread. Here's an SD adapter for the PSP:
Around $26 shipped.
What's noteworthy is that it uses up to 2 cards at the same time. Someone successfully used it and posted a video in YouTube using 2X4 gigs - 8 gigs of memory. You have to format and transfer through the PSP for it to work.
Here's a 4 gig SD card for $23 after rebate:
The other thing about the adapter is that if you use 2 cards and you take them out, you have to make sure to put them back in in the same order (the one on top on top again, the one on the bottom, on the bottom again.
I ordered my adapter yesterday. Can't wait
It has been widely used and reported about on gaming forums.
For a quick comparison of current prices, you can get an 8 gig SDHC card for $71, and an 8 gig MS Pro Duo for $150.