There really is not much to add to the instructions on the Linux Clues blog

• The chroot needs to be somewhere on an ext3 partition, so you should have a large /home partition or an SSDcard with an ext3 partition
• I started with the current-stage3-armv7a_hardfp tarball. Take care to preserve owners and permissions when unpacking (I forgot that the first time...)
• I made a little script to mount essential directories like /dev /proc ... into the jail, entered the jail through the "front door" (sudo chroot $JAIL /bin/bash), downloaded the portage tree and started emerging
• After setting up a few essential things (for me: zsh, emacs, perl) I made a startup script that starts the jail with only xinetd running.
• Even sshd is run by xinetd, so that an uninhabited jail only has one small process running.
• I inhabit the jail as an unprivileged user with the same UID and GID as 'user' on the N900. This makes sharing files and using dbus much easier.
• Gentoo takes a lot more space than other distributions: now already 7 GB with only a few X applications built
• Gentoo has a particularly easy way to setup cross-compilers (I just ran 'crossdev armv7a-hardfloat-linux-gnueabi' on my main machine). A cross-compiler is necessary to be able to speed up builds with distcc.

Because of the last point, I suspect it is more work, but still doable, for non-Gentoo users

I added a screenshot, not much to see there, really. X programs run full screen under the Hildon window manager, there are no fancy desktops. It shows my favourite system monitor, xosview. Because /sys and /proc are mounted under the jail it has access to all the info it needs.