Booting from external SD card:

Why, because you can set the system partition to 1GB instead of the stock size and install, and install, and install without issue No more fighting for space. Also, and the primary reason for me doing this, is that once I get said SD card set up exactly the way I want, I can make a bit-for-bit copy of it and get my 100% backup file. Never have to worry about it again. I can also move said SD card between N810s and everything will go with it.

Lots of good reason, actually.

This worked for me:
(from: http://talk.maemo.org/showthread.php?t=25714)

But I had to do the manual swap thing as specified here:
(from: http://talk.maemo.org/showpost.php?p...&postcount=168)

Worked well, running of an 8GB SD card and seems very stable. No issues noted so far. For now, I've put the swap and OS on the same card, which is not ideal, but I'm just experimenting at this point.

David...