Likewise when you attach the charger the phone boots up enough to run the charging software.

I'm sure that the system has a real time clock chip (maybe a DS1337) that only pulls a few microwatts when the rest of the phone is powered down. The alarm software sets the next alarm time on the real time clock chip. That chip will signal an interrupt line and will also signal the voltage regulator to power up the device. The phone on startup notices that the alarm fired and will notify the user. Any phone with the right hardware should be able to do this.

That's pretty much it. The tablets have had this feature since the N800 days. Basically what happens is when you schedule an alarm with AlarmD, it registers the timing of the event with the realtime clock (the N8x0s used to use retutime to do this, not sure if that applies to N900 or not). The clock chip is running always, even when the phone is off; once it detects that it's time to wake up, it triggers the device to boot up into a special "act-dead" mode (as Nokia calls it) which then reads the alarm queue to check to see what triggered the boot, and display the alarm stuff.

There's nothing special about "Act-dead" mode for the alarm, other than the fact that it's coded into most of the bootup scripts that if the device is triggered in that mode a lot of those will not run; so the device "starts" quite quickly because it just ignores a bunch of normal bootup proceedures. The downside to that is that if you want to keep the device on after the alarm, it has to actually restart in order to properly process all of that "normal boot" process.

The coolest part about it all really though is that you can use the AlarmD interface to boot "normal" mode and do things from power down as well... it's not a feature of the built-in alarms application as far as I know... but proper usage of the AlarmD api allows it.

Take a look at FlipClock for example. We use the AlarmD api to register alarm events... so if you set an alarm within Flipclock, turn your device off... when the time for that alarm comes up, the device automatically boots, launches FlipClock, and triggers your alarm! That API and custom ability is what makes the whole alarmD thing so cool/unique on the Nokia devices...

(btw for the record, the battery charging is actually an even more stripped down "act-dead" mode... the device actually powers up, loads frame buffer drivers for the display/etc, then shows you the charging screen).