The live-installer (ubiquity) displays that at least 2.6GB of disc space is required for installation so I filed a bug report during Beta1 iso-testing:
Please report your actual installation size including SWAP. It would be best to report each partitions size related to that install.
I'm quite confident in saying that anything less than 5GB for / is a disaster looking for a place to happen, unless they intend for you to try it, hate it, and remove it
The replies from the devs would certainly make one wonder if they even use Ubuntu themselves
Here's a screenshot:
I think 10GB minimum is more realistic, and still a bit on the low side. The stated 2.6GB is just nuts!