Unfortunately what you said is true, even I could not get this resolution on my DQ45CB, although the GFX chipset can support resolution higher than that, it is very strange why it is not possible to get this particular resolution.
Hopefully, someone can explain this to us, as I heard that on certain boards this will be fix possibly thru either a bios fix or gfx driver.
Let us wait and see.
All the best,
I had the same problem before, so i explore on any possibility, and finally found a way that work.
Right click on your desktop, choose graphics properties...
go to display setting, set your refresh rate to 75 Hertz
and voila, appear 1440 x 900 on screen resolution option.
hope that could help.