Have a look at this page:
nexusonehacks.net/.../how-to-install-ubuntu-on-your-nexus-one\
android/

Reportedly, this route works for a few other android phones. I'm not
sure how legit, but the developer quotes "Once you have a “rooted”
Android phone, you will be able to run Ubuntu under Chroot. " Then he
goes on to describe it as basically running ubuntu as a virtual
machine on your android device. I'm note quite
technically-intelligent enough to know if the article is legit, or
just somebody running remote desktop looking for their 15 minutes.
But it seems OK. Maybe someone with more knowledge on android can
check on it.

I know that you were looking for dual boot, but unless you want to
switch to windows mobile, this looks like the only option for ubuntu
on android right now.

Here also is one link for instructions on running debian:
www.talkandroid.com/.../1091-install-debia\
n-android.html

Being a WinMo user, I still cant say if this is actually a dual boot
option, but it appears to be. Google "run debian on android phone"
and have a look for yourself at other options. I'd be interested to
know if you try any of these how it works out. I've been thinking of
switching to an android phone for a while now, but have been hesitant
as I am a big fan of all of the development on porting various OSs on
my current Tilt 2 windows mobile phone. I am a sucker for tinkering
with mobile devices. :) I just lack the programing knowledge to
participate in the development myself.