I bought mine N900 in November/December 2009 and also have the "dark keyboard syndrome" actually I never noticed it untill today :O I have second N900 "just in case" bought in 2012, taken out of box few times only to check if it's still working, otherwise resting in a box sealed in additional plastic bag - however I strongly believe it was not new but refurbished - and suprisingly it indeed has brighter keyboard (or maybe it's placebo effect as the contrast is not as big as in wpwrak's pictures, however the illumination seems more consitent across all keys and more white'ish).

This is the second time I WTF about the hardware I carry in my pocket every day - first one was also connected with Neo900 developement I think (or at least with joerg_rw), it was the fact that N900 had small internal battery to keep the date/time settings when taking out the main battery.