some of you might never have known: N900 is not supposed to lose cmos-clock time and date ([edit] and alarms) on every battery swap. It's a tiny rechargable coin LiIon cell backup-battery (next to vibrator and power button) that's supposed to keep RTC aka cmos-clock running during months of main battery removed. Alas all those components break during a 12..24 months of usage, some even leak.
Now for the "bright" side of this story: the battery is easily replaceable when you got some moderate soldering skills (and tools), and using a
YUDEN, PAS414HR-VA5R, capacitor, 0.06F, 3.3V, STAKED COIN Coin Type Polyacene Capacitor https://hbe-shop.de/Art-2112777-TAIY...3V-STAKED-COIN will avoid having it broken again in another 12 months of time. (datasheet)

[edit]my local stock of those ran out, so the following doesn't apply anymore[/edit]
For your convenience I got some of those components as of above, and I'm willing to ship them worldwide via simple letter, for a 3EUR donation per unit to maemo donation paypal account. Contact me via IRC(freenode.net):#maemo or by mail to techstaff at maemo.org, or simply donate and give FULL addr plus your online contact (mail or IRC) in "message to account owner"

some macro-shots (taken with N900):
pointing to coin cell with a red balltip fine pen left black switch is camera trigger, right one (white head) is power button. The huge silver thing below the coin is vibrator also see http://maemo.cloud-7.de/Gallery-N900-exploded/

cheers
jOERG