Just to respond to myself here: I've gone ahead and tested a bit, and the 0-1023 range for the V4L2 control appears to be valid for the N900's main camera. (I haven't tested with the front camera...)

I should note that, when you provide a focus value that is far away from the current value (say, 200 or more points distant), the camera makes an audible "click" noise as it changes the focus. I don't know if this is standard, but it kind of makes me nervous! For now, I'm first querying the current focus value (via VIDIOC_G_CTRL), and then incrementing (or decrementing) it stepwise by small amounts.

I haven't found any yet, but are there any reports out there of damaging the N900 camera by trying to change focus too quickly?

EDIT: Aha, yeah, this is definitely a problem: Bug 9085. Long story short, Maemo's camera focus driver moves the focus too quickly, and therefore can damage the camera. So, best to never assign values to V4L2_CID_FOCUS_ABSOLUTE that vary significantly from the current value.

Also, I should note that after closing my app, I can hear the camera make that clicking noise; so, it seems that you'll also need to reset the focus back to its original state (carefully!) before letting go control of the camera, if you're playing with manual focus.

EDIT 2: I decided to play with the camera app on my stock, non-development N900 to see how it worked. And yeah, I can still hear that "click" noise when it moves the focus level around. So, maybe there is no way to avoid this problem? (Does the CSSU have a fix for this bug? I never know how to figure out exactly what fixes are in the CSSU... )