iFinity

Well, the page index is cached in the DNN cache.  So the question is really : what sort of caching are you using for your web farm?  

In the DNN Web Farm guide, it states that "The end result should be that any 3rd party modules that use the core API for managing the cache will automatically support web farm cache synchronization." - this is how the Url Master module works.  It uses the core API for managing the cache, so either using the broadcast polling provider or file based caching provider should provide notification to the other webservers in the farm that the cached item has changed.

I don't have any recommendations for the page index cache settings for Web Farms as each installation is different.  However, if you're adding new Urls, you really want the cache flushing to occur straight away : otherwise you'll get inconsistencies across the different webservers.

I'm not an expert on web farms, and I do know of some installs using the Url Master module in a web farm situation.  If you can install any cache monitoring tools this will help see if the page index cache does get flushed/rebuilt and notified to other servers.