You don't have to (and BETTER NOT) update to 2.3.7. Trust me, it's HIGHLY EXPERIMANTAL and VERY UNSTABLE.
But what if you want the camera? Easy, just download the N12_update package mentioned up there, and use WinRAR to browse the package, get into system/lib, extract the 3 camera related .so library files somewhere on your N900 (libcamera.so, libcamera_client.so, libcameraservice.so)
[EDIT: 3 files posted below in zip package!]
On N900 use FileBox or whatever means to copy the 3 files into /home/system/lib. Reboot and camera will work.
(Actually that's one step from how I blended 2.3.4 and 2.3.7 and make most functions work on my device!)
Be aware though, the camera is still in early development, light sensing is bad (dimmed photos), no autofocus and even no zooming!