If you downloaded an older image than the current one, you shouldn’t need to reinstall, but be sure to review the changelog in the link above.
Note that this blog post originally contained a bunch more information, which has been moved to a dedicated page on wiki.ubuntu.com.
I’ve closed comments on this blog post. If you are looking for help, please see this post on the raspberrypi.org forums. If you post there, you’ll be reaching a wider audience of people (including myself) who can help you. Thanks for all of your comments!
After my last post, I went and ported Sjoerd’s Raspberry Pi 2 Debian kernel patchset to Ubuntu’s kernel package base (specifically 3.18.0-14.15). The result is an RPi2-compatible 3.18.7-based kernel which not only installs in Ubuntu, but has all the Ubuntu bells and whistles. I also re-ported flash-kernel based on Ubuntu’s package, recompiled raspberrypi-firmware-nokernel, created a linux-meta-rpi2 package, and put it all in a PPA.
With that all done, I decided to go ahead and produce a base Ubuntu trusty image. It’s 1.75GB uncompressed so you can put it on a 2GB or larger MicroSD card, and includes a full ubuntu-standard setup. Also included in the zip is a .bmap file; if you are writing the image in Linux you can use bmap-tools package to write only the non-zero bytes, saving some time. Otherwise it’s the same procedure as other Raspberry Pi images.
(PS: If this image becomes popular, I should point out ahead of time: This is an unofficial image and is in no way endorsed by my employer, who happens to be the company who produces Ubuntu. This is a purely personal undertaking.)