I'm interested in WSXGA+ only (1680x1050) and was looking for informations about LED backlight in this type of the screen - I have found nothing (and was looking in many laptop's brands) - I suppose the technology doesn't allow to make it with the acceptable cost.
I don't think that's the case. I found the new Dell workstation M4400 (15.4") is offering the following displays:
15.4" WUXGA TrueLife RGB LED display (1920x1200) resolution (Coming soon.)
15.4" WUXGA AG day light viewing (1920x1200) (Coming soon.)
15.4" WXGA+ AG WLED Back-lit display (1440x900) resolution
15.4" WXGA AG display (1280x800) resolution
So I believe 15.4" WUXGA 1920x1200 LED display is possible.