The problem:

I got a new 20" widescreen lcd monitor (Acer AL2016W), but was unable to get X11 working with the monitors native resolution of 1680x1050 (common typo 1650x1080).
This was suprising as I thought my video cards, Radeon 7000/VE and a Radeon 9200 SE, should be able to handle this resolution.

The solution:

After much googling, I ultimately discovered the problem was simply that X11 did not know the correct parameters for this mode. I then found out that X11
conveniently spits out available mode details when it's starting, you can find the info in your x startup log, probaly in /var/log/XF86Config-4.log.
 I took the modeline being outputted when starting X and inserted it into the Monitor section of the XF86Config-4 file, and voila, native resolution.


The following is my monitor section, notice the lines that are #Commented out are taken directly from the x11 startup log. 
The "ModeLine" entry uses the numbers taken from those commented lines.  ModeLines first parameter is the name of this mode, which is used later 
in the "Display" subsection of "Screen". The next parameter is the clock value in MHz, and the remaining values are taken directly from the lines
 starting with h_active and v_active. All of the values from both h_active and  v_active are listed, each seperated by a space, with all
 the "h_" values listed first, and the "v_" values listed second.  If you find this confusing, see the diagram I made below.  Once you've set up your 
modeline, simply set your newly created mode to be the default mode in the "Screen" section.

-----

Section "Monitor"
        Identifier      "ACER LCD"
        HorizSync       31-84
        VertRefresh     56-77
        #(II) RADEON(0): Supported additional Video Mode:
        #(II) RADEON(0): clock: 146.0 MHz   Image Size:  433 x 271 mm
        #(II) RADEON(0): h_active: 1680  h_sync: 1784  h_sync_end 1960 h_blank_end 2240 h_border: 0
        #(II) RADEON(0): v_active: 1050  v_sync: 1053  v_sync_end 1059 v_blanking: 1089 v_border: 0
        ModeLine     "1680x1050" 146.0 1680 1784 1960 2240 1050 1053 1059 1089
EndSection

----


ModeLine Explained:

ModeLine "Name" [clock] [h_active] [h_sync] [h_sync_end] [h_blank_end] [v_active] [v_sync] [v_sync_end] [v_blanking]

Diagram:



                                  #(II) RADEON(0): v_active: 1050  v_sync: 1053  v_sync_end 1059 v_blanking: 1089 v_border: 0
                                                              |       _____/  _______________/                /
                                                              |      /   ____/  _____________________________/
                                                              |     /   /      /
          ModeLine     "1680x1050" 146.0 1680 1784 1960 2240 1050 1053 1059 1089
					   |    |   |___  \________________
                                           |    \______ \_______________   \______________
                                           |           \                \                 \
              #(II) RADEON(0): h_active: 1680  h_sync: 1784  h_sync_end 1960 h_blank_end 2240 h_border: 0

			


2016 update:
Achieving the same thing with XrandR:

xrandr --output VGA1 --mode 1680x1050 --rate 146.0