Because archive.ubuntu.com hasn't any IPv6 endpoints, an IPv6-only Ubuntu machine is quite unusable: no package management, no updates, etc.
Is there any way to overcome this problem without enabling IPv4?
The main archive server, and many mirrors don't support IPv6 so going IPv6 only is very cutting edge of you but... most of the rest of humanity probably will join you when we are well into 2012 (if not 2015) or when it becomes really unavoidable.
That being said...
So the answer is yes you can do it w/o IPv4 but why not use both IPv4 and IPv6 like everyone else does that uses IPv6?