A dd command gives:
# dd if=/dev/null of=/dev/tty0
0+0 records in
0+0 records out

To test the framebuffer allocation you should do the following:

dd if=/dev/fb0 of=/dev/null, so that you are actually writing from /dev/fb0 to /dev/null (not from /dev/null to /dev/fb0 as you did)
Then the complete framebuffer will be dumped to /dev/null, and the amount of records dumped will be shown.
The result should be 150+0 records for your application (240 times 320 times 1 byte (8bit color) divided by 512 (standard blocksize) = 150 records)

For the rest, I can't really see anything wrong with the pxaregs. I assume you already tried playing with pixclock polarity and that you checked with the oscilloscope that you actually receive good signals on the correct LCD pins?

Regards,
Bas