Can the list controls not be upgraded to do this themselves?

i.e. if the list is short, no scrollbar/letterjump
If the list is medium length (up to X pages?) show a draggable scrollbar
If the list is long, show the letterjump

The list knows itself how long it is.. On most dektop platforms lists render their own scrollbars depending on their own length, and the developer elects to have the list show scrollers or not. The principle could be replicated in the N900s ui