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...

1. I found a link from 2008 that states you can use a gateway from sixxs.net
   From the link:

   
   
   

   
   

   Edit your sources.list and replace http://archive.ubuntu.com/ubuntu/ by http://archive.ubuntu.com.ipv6.sixxs.org/ubuntu/

   
   

   
   
   

   Looking at the website this looks like it is still working.




2. Some mirrors are IPv6-ready (the ones operated by BIT). Their URLs are...

   
   
   
   
   
   • nl.archive.ubuntu.com / ubuntuarchive.bit.nl
   
   
   • ftp.belnet.be (BelNet)
   
   
   • ie.archive.ubuntu.com / ubuntu.ftp.heanet.ie (HeaNet).
   
   
   • mirrors.xmission.com (operated by Xmission)
   
   

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?