I created a VMWare image with Ubuntu 10.04.3 to use for building OpenJDK 8.
With a clean Ubuntu install, including latest updates, I followed this guide: http://hg.openjdk.java.net/jdk8/jdk8/raw-file/tip/README
1. Installed mercurial
apt-get install mercurial

2. Created dir "Projects", in which I cloned the OpenJDK Mercurial repository:
hg clone http://hg.openjdk.java.net/jdk8/jdk8 JAmigaOpenJDK
cd JamigaOpenJDK
sh ./get_source.sh

3. The readme on howto install the Bootstrap JDK tells us to install JDK 6. However, JDK 8 needs JDK 7. This turned out not to be as easy as "aptitude openjdk-7", since the Ubuntu 10 aptitude repository doesn't contain JDK 7. Google helped me find: instructions on how to do it. You apparently should download the thing from Oracle directly. So:
3.1. Download Java 7 SDK from Oracle.
3.2. Untar and move to /usr/lib/jvm
tar xf jdk-7u3-linux-i586.tar.gz
mv jdk1.7.0_3 /usr/lib/jvm
export LANG=C ALT_BOOTDIR=/usr/lib/jvm/jdk1.7.0_3

Now the sanity check should complete ok (remember to CD to JAmigaOpenJDK):
make sanity
...
Sanity check passed.

4. FreeType needs to be in version 2.3 or newer. Running freetype-config --ftversion said version 2.3.11 was installed. Must be good, right?

5. Apache Ant v 1.7.1 is also required, which already is available in my Ubuntu.

6. Next, fire away a make all and patiently wait.
make all

7. The build/linux-i586/j2re-image directory contain the stuff built.

in order to ease my development I downloaded a JDK branch: hg.openjdk.java.net/jdk8/jdk8/jdk/rev/0b052b7f3f83 using the ZIP-link at hg.openjdk.java.net, jdk8/jdk8/jdk. This contains the bsd/linux/macosx/solaris/windows and share stuff. This should be the place to put JAmiga specific stuff. My aim is to build this on the Amiga, and use j2re-image. I'm assuming this won't work in the long run; I'll probably have to change some stuff in my JAmigaOpenJDK-ubuntu install and rebuild to get everything correct.