I was having the exact same problem under the following system:
Host OS: Win 7 Professional 64
Guest: Ubuntu 9.10 32bit
Dell Vostro 1520 (core 2 duo p8600, 4Gb ram, NVidia GeForce 9300M 256mb)
After reading the bug tracker entry ( http://www.virtualbox.org/ticket/6254
) i tried increasing the video memory from the default (12mb) to 32 mb.
It´s fine now, just posting this as a possible solution. Maybe the bug tracker should get an update