The SoCs (System-on-Chip) that Nokia and others use have a small piece of code and a set of keys in ROM to check the authenticity of the bootloader. The bootloader then checks the authenticity of the firmware image (e.g. Symbian OS), which in turn checks the authenticity of each piece of software to be installed.

The check in Symbian OS has been defeated, but the other two checks have not been, to my knowledge.

OK so just needs plenty of computer time and brain power.

In other words... not impossible... Just highly improbable

the HTC universal was a brilliant piece of hardware, and at one point looked as if it was going to take over from the Zaurus as a pocket computer with keyboard as the Uni had wifi and bluetooth and phone features, when the Zaurus only had the CF slot.

The Uni came with Windows Mobile and even though there was almost no documentation people found a way to make it run linux. Progress was painful, but they managed pretty well.
http://handhelds.org/moin/moin.cgi/Universal
Unfortunately by the time linux ran fairly well on it, the world was moving on and the universal was dropped from HTC's line and Android was imminent (very rough time lines).

So, yes, in theory it can be done, but it's very hard to get linux to run really well on a device so that all its hardware is supported... and quite often when the device's makers realise their device is under scrutiny, lock it down even more - see ipod linux projects.

http://www.nopcode.org/0xFFFF/?p=docu&q=faq - Shows some pretty interesting information about the N770/N800/N810´s bootloader

Does the N900 also have such cryptographic locking down of the boot sequence? I hope not, as this would contradict the entire philosophy of the "open device" that I've been hoping the N900 would deliver.

Will it be possible to flash my N900 with another operating system, should I so choose? Can I compile Maemo from source code, make some tweaks, and flash that? I had assumed the answers would be yes (just like with my trusty old Zaurus), but now I'm a little worried...

You can run other (linux) operating systems on the N900, like Debian, Mer, Ubuntu, probably Android, etc... AFAIK, you can't "compile Maemo" because it uses closed sourced apps. The open part of Maemo + community parts = mer. I mean you can flash the kernel, change all the **** you want, get root access in one second, etc...