Since my previous post, I've found some modelines that will work in Xfree86 and X.org...

In the monitors section, set horizontal sync to 31.5-100
# HorizSync is in kHz unless units are specified.
# HorizSync may be a comma separated list of discrete values, or a
# comma separated list of ranges of values.
# NOTE: THE VALUES HERE ARE EXAMPLES ONLY. REFER TO YOUR MONITOR'S
# USER MANUAL FOR THE CORRECT NUMBERS.

HorizSync 31.5-100.0
Vertical refresh works between 30-90, and below that are some modelines I dug up. The actual sizes of most 15.4" widescreen LCDs are 1280x800, 1680x1050 and 1920x1200, but the other modelines should also work (KDE will squish/stretch as needed)

# VertRefresh may be a comma separated list of discrete values, or a
# comma separated list of ranges of values.
# NOTE: THE VALUES HERE ARE EXAMPLES ONLY. REFER TO YOUR MONITOR'S
# USER MANUAL FOR THE CORRECT NUMBERS.

VertRefresh 30-90
ModeLine "1920x1200" 162 1920 1984 2176 2480 1200 1201 1204 1250 +hsync +vsync
ModeLine "1680x1050" 147.14 1680 1784 1968 2256 1050 1051 1054 1087 +hsync +vsync
ModeLine "1280x800" 84 1280 1344 1480 1680 800 801 804 829 +hsync +vsync

EndSection
Then you need to put those new modelines into the 'Screen sections' section. It will look something like this (fyi, as described it will boot into 1920x1200, 24 bit depth because that's the first option for the default depth)

# Any number of screen sections may be present. Each describes
# the configuration of a single screen. A single specific screen section
# may be specified from the X server command line with the "-screen"
# option.
Section "Screen"
Identifier "Screen 1"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 24

Subsection "Display"
Depth 8
Modes "1280x1024" "1280x800" "1024x768" "960x600" "800x600" "640x480"
ViewPort 0 0
EndSubsection

Subsection "Display"
Depth 16
Modes "1920x1200" "1600x1200" "1680x1050" "1400x1050" "1280x1024" "1280x800" "1024x768" "800x600" "640x480"
ViewPort 0 0
EndSubsection

Subsection "Display"
Depth 24
Modes "1920x1200" "1600x1200" "1680x1050" "1400x1050" "1280x1024" "1280x800" "1024x768" "800x600" "640x480"
ViewPort 0 0
EndSubsection
EndSection
These will of course only work on Hard Drive installs where you can change /etc/x11/xfree86.conf