Until Hudson 1.345, SCM implementations had to implement boolean pollChanges(...) in order to indicate if a build is necessary.
In 1.345, it has been changed to a two-phases polling which enables to store the repository state after checkout, and after each polling.

Specifically, for ClearCase UCM, it allows to store foundation baselines information, and also the last polling time.

It means :

• change of foundation baseline (rebase) can be detected and can trigger a build
• polling will only consider changes since last polling instead of last build
• changes on the view due to rebase with no conflict can be reported in changelog