Spring is finally available through Maven repositories for final releases, milestones and snapshots, and they are adding the sources too! Ben Hale has listed the repositories that you need to have into account.
- final releases available through central maven repo (no need to do anything at all)
- milestones through http://s3.amazonaws.com/maven.springframework.org/milestone (browse)
- snapshots through http://s3.amazonaws.com/maven.springframework.org/snapshot (browse)
I personally would like to see the milestones in the central repository as it makes life easier for users as many other projects do.
Carlos Sanchez's Weblog 
The decision to keep the milestones out of the central repo was not one that we made lightly. In the end it came about as a response to the lifecycle of objects in the central repo.
Any object that is placed in the central repo is there permanently and this is a *really* good idea. However, when we look at the lifecycle of milestones we don’t want them to last forever. At some point (in our case after a release plus a grace period) we want those objects to die. This is the same reason that we don’t put snapshots there, right?
For us, milestones allow people to get a look at what’s coming in the release and be prepared, but we don’t want them to stay on the milestones and we don’t want to support them if they do (especially when bugs are fixed in the final release).
So for the Spring Portfolio, this was really a way to adjust the contract of our artifacts without needing to change how the central repository worked.