Its always been handy to issue the occasional "analyze index ... validate structure" to get statistics from INDEX_STATS to see the current state of any index. The problem was finding a time to do it without hurting the users.
9i now lets you do it online in the same way as an online rebuild, namely, all you need is a "quiet" moment at the beginning and end of the operation.
analyze index MY_INDEX validate structure ONLINE;
Looks pretty impressive eh?
But one BIG obstacle. The online operation is designed purely for structure validation. The INDEX_STATS view will not be populated.