UbuntuChroot
It is possible to use a disk image instead of creating a partition. However, this comes with a few caveats, such as slower speed, disk size restriction (mounting a disk image in cryptofs may limit you to < 2GB) or breaking USB connectivity (on /media/internal). Do this at your own risk. Partitioning with Meta Doctor is the faster and slightly stabler method.
You need a large-enough ext3fs partition created with Meta-Doctor mounted at /media/ext3fs (instruction how to create partition is available here Chrooting). Note that the latest optware bootstrap package will also use this if it exists. If you have not already done so you should setup the ext3fs partition to mount at bootup. Instructions can be found on the Meta Doctor page.
Make sure you have the WebOS Internals Testing Feeds enabled.
Using Preware, install Xecutah and Ubuntu 11.04 Chroot.
Hit the relevant buttons on Xecutah to run the XServer and the Ubuntu 11.04 Chroot.
Use "apt-get install" to install whatever apps you like from Ubuntu. They run in the Ubuntu chroot and render on the XServer.
Don't forget the donation link on the Main Page, and please install the Preware Homebrew Documentation app from the official app catalog to learn how to install Preware on your device.