The image above contains a new kernel, using the old kernel will probably work, but you might miss some features added in by Ubuntu.
I have NOT been able to boot the new kernel yet.

To install the image, download and extract the raw file, then plug in your N900 and select mass storage mode, find the name of the dev node (/dev/sdx) then dd if=imagefilename of=/dev/sdx. You have to dd to the whole mmc block, not the partition (i.e sdc not sdc1).
If you type in the wrong block, you might erase your HDD, so make sure you know what you're doing, dd isn't something to try out.