When I try to install Maemo's man-db, which I thought might
let me view man-pages, on my N900, apt-get says it's going
to remove 283 MB of what looks like around 300 programs,
including adobe-flashplayer, busybox, calendar,
clock-ui, dbus, maemo-launcher, microb-engine,
mplayer, opera-mobile, rootsh, upstart, and udev.

What's going on? "apt-get -f install" checks OK.
My sources.list looks OK.
Should I go ahead and try?

No!

(Incidentally, please name some man-page viewers).
TFJ

Yeah go ahead if you like to break your device completely.

Post a bit more info.
I have man-db without problems on my device.

You know about repo mirrors and use them?

I believe I'm using the basic maemo repos. apt-get just says this is what it's going to do. It also lists a bunch of 'osso', 'hildon' and the like programs for deletion.

man-db's apparently in the SDK repo. apt-get wants to remove 380 files.

I sometimes prefer apt-get to the manager apps.
The N900 has seemed vaguely buggy (slow, occasionally freezing) lately.

Basic maemo repos are okay.
But Nokia repos are gone (search here for maemo-repos.com or maemo.muarf.org).

Nevertheless I should read questions here really more thoroughly before answering.
Do NOT install man-db from SDK but install package man-db-n900.

Not sure about the details and I have little time to search my own posting history but I've had the exact same problem with man-db on my N900. At some point, it must have been close to midnight, I decided to remove it, and without looking closely I had apt delete everything it found on it's way.

SOLVED

Correct, the man-db-n900 package, in the extras-devel repo, installed
fine (but with lots of "dangling symlink" warnings, dang it) and seems to work great.
What happened is, when I searched (with apt-cache) for "man page" or such, I had left the SDK repo open in my sources.list file, and grabbed at "man-db" (apparently I missed seeing "man-db-n900").
So this may be a good example of why you should stick to the Extras repo, and be real cautious with the others; this problem was pretty obvious, but others might not be.

Perhaps you could put the 'Solved' tag to the title thread as well?

And please, for forms sake, let us note that apt-get IS NOT A RECOMMENDED way to install things on the n900.

It can bite you HARD, unless you know wots wot and are attentive.

???
I do install almost everhthing via console ...

Agreed.

But having HAM deciding in the background is not an option either ( for me).

Yes, but that after-all is YOU :-)

I believe my advice is that recommended for most users of the n900.