Actually, the repositories will not disappear in 2012, since the server support remains for 5 years - April 2015.
Support for the desktop disappears in April 2013 - LTS releases are 3 yrs for the desktop and 5 years for the server. From 12.04 - both the desktop and server support arrangements will be 5 years.
Once the desktop support ends, it doesnt mean the repositories or desktop packages are removed - it will just mean that canonical will not actively update them with stability or security updates.
When the server support ends, it is likely that the repositories will be moved to the old-releases area of ubuntu.com - as described in the community wiki.