So tonight i fully flashed my N900, Vanilla and Fiasco.
As a result, I have a brand new machine, but still no wifi. The dmesg pattern is the same:
wl1251: down
wl1251: 151 tx blocks ...
wl1251: firmware booted (Rev 4.0.4.3.7)
wlan0: direct probe to ....
wlan0: direct probe to ....
wlan0: direct probe to .... timed out

So I guess I should conclude that my N900 has a dead Wifi.
Do you have any clue if I bring it to a shop (somewhere in Zürich), they'll know what to do with it?

Considered the time I'm spending fighting with it, I'm also good to buy another one, if anyone is selling his.

Thanks sixwheeledbeast for following.

jb