I had a similar experience when i had my n95 when i dropped it off 5 times to a Nokia center till i spoke to the manager and proved to him in simple terms how there is in fact a problem. we had a lot of arguments and boy they had red faces. I understand how you can have bad feelings towards nokia/n900 as i did with my n95, but when i had to get rid of my n95 i will never forget the experience i had with it. loved the good and bad

just tell them this. keep it simple as the guys at Nokia Centers dont think outside the box usually. cooks not chefs

1: Ask to speak to the manager
2:show him what you just said with the ckfs.vfat -wa scan
3:show him Conky as evidence there is infact space on the internal drive and rootfs, and show him the memory through settings panel.
4: show him you have re-flashed the device, to prove it show that your n900 was impossible shipped with V3 firmware since it wasn't out yet and its a month old lol.
5:Then try to install an applications and when it fails how can he deny there is a problem???
6: call Nokia support, nokia the center and tell them what happened with the screwing around at the Nokia centre and im 100% they will give you 3 months extra warranty (Trick for everyone, they give it out with any complain keep that on the down low)

good luck. I did a simliar then and it worked flawlessly.