SAK-12253. Quoting Ray from there:
"Aside from the desirability of keeping dependencies up to date, Spring's new annotation support seems likely to speed application development, and new features such as Java-based configurations and OSGi-compliant framework packaging may prove useful in our kernel / framework cleanup.
As usual, the Spring team promises good backward compatibility support. Any glaring exceptions that turn up should be noted as comments on this task."
My personal interest in this upgrade is to enable support for grails as a fully-supported tool development stack for Sakai (that's a separate issue, but the Spring upgrade is a prerequisite).
I have patches ready for sakai trunk and kernel trunk.