I've search but can't find much. Does the N900 have an internal battery for time / settings functionality like on a pc?

My e65 doesn't, but a few of my friends phones do.

As far as I know, most, if not all, modern phones have a small battery internally to maintain date/time/settings (similar to how PCs have a cmos battery to keep BIOS and time settings). Otherwise, every time you take out the battery or when your battery drains completely, the time/date (and possibly settings if it isn't saved onto memory) are wiped.

More likely to have a capacitor to store the time on battery changes, though I'm not an expert.

Certainly there's no way to change any purported backup battery, which makes me think there is probably not one.

on my htc wizard, there probably is a backup battery - however now, after a few years of phone's life, it's depleted (either that, or the capacitor went dead somehow) and each time the battery is taken out, clock is reset to factory original value.. quite annoying - the capacitor would be much more useful, if it lasted a hours, that is...

Yeah it is annoying. My E65 never had one, pain if you have multiple batteries

Some do, some don't: my wife's old 7610 didn't.

There's an easy way to find out: pop out the battery out of an n900, leave it 30s and put it back - if the clock's accurate it has at least a capacitor.
That of course, if you can live with your new and shiny n900 off for 30 secs.

yeah id love to know this, and will be one of the first tests i do, as my Axim resets every time i change a battery and its a real pain.
I really am not worried about bettery life if you can change battery without issues.

The N900 has a button cell battery. See the second photo on the first page (PDF link) near the top center.

Thanks. Obviously not user-changeable, so it will eventually die, but hopefully not for a few years yet.

Also note that the N900 can automatically update its clock from the network, presumably via NTP. Everything else is stored in flash memory that doesn't need a battery.

With out-of-the-box software, the N900 can automatically set its clock via information from the cell provider, like most (all?) phones. Network time protocol daemon (ntpd) is separate download from Extras/Extras-testing (I haven't checked which), but should be an easy one. It was always install-and-go on previous Maemo devices, and I don't expect that's changed with Maemo 5.