I have successfully reflowed the dreaded Nvidia video cards in three laptops, and they continued working for 2-6 months afterwards. I did not bake the motherboards. I covered them completely in tin foil, leaving only the video chip exposed. Then I used a heat gun to reflow them (there is a great video on youtube of a thinkpad being reflowed using this technique). But the situation in your case is both better and worse at the same time. These laptops still had the video chips attached to the PCB, which made reflowing easy. However the original manufacturing defect was still present which is why the fix was only temporary. If you manage to line up the chip with the PCB and then you reflow your N900, the chance is pretty decent it will work as new.