I have a button on my toolbar for "Rebuild index". I have used it in the past. Under beta 8, when I click it, the little blue logos icon flashes very quickly and then exits. I just tried typing Rebuild Index into the command bar, waiting until the drop down shows it, and then I click that, and the reindex has now started and is working properly!
I have one machine reindexing, and one machine building an index from scratch. I'll do more investigation after they are finished.