I got a SanDisk Pro Duo that was very cheap over Amazon, so far demo downloads from PSN have worked without any problems, and I like SanDisk in general. That´s all I know about that matter
Just got a Sandisk Pro Duo 4gb from allmemorycards.co.uk and both it and GT PSP should arrive around my birthday (27th - maybe Play will ship it early). Don't have a working memory card until then.
In france you can get memory sticks pro duo 8gb for 35 euros... sony and fully working
just don't go in the video game square of a big shopping center.... go to memory cards lol