Quoting at length from the UbuntuBackports page,
"Ubuntu releases a new version of its OS every 6 months. After a release, the version of all packages stays constant for the entire 6 months. For example, if Ubuntu ships with OpenOffice.org 2.0.x, it will remain at OpenOffice.org 2.0.x for the entire 6-month release cycle, even if a later version gets released during this time. The Ubuntu team may apply important security fixes to 2.0.x, but any new features or non-security bugfixes will not be made available.This makes a lot of sense.
This is where Ubuntu Backports comes in. The Backports team believes that the best update policy is a mix of Ubuntu's security-only policy AND providing new versions of some programs. Candidates for version updates are primarily desktop applications, such as your web browser, word processor, IRC client, or IM client. These programs can be updated without replacing a large part of the operating system that would affect stability of the whole system."
Looking at the ubuntu repository package page for blender, one can see that feisty is "stuck at" 2.43, but feisty backports allows one to get blender 2.45. Instructions that ubuntu provides for adding backports are shown here.