Very interesting idea.. I like giving my members options for their new posts search, I used a mod on vB and on IPB I am using their Today's active content, which gives my members a chance to change the date of their search.
Something like this built into xF would be great in my opinion.
Some forums only show what you haven't read - but if you read it then it disappears from the list. Should you want to go back to that thread later, or see data that has been added in the past x days - I just marked all forums read have no notion any more of data as it came in and look or see topics with out replies.
Other forums just show you the past 24 / 48 / 72 hours etc. but all that does is add more posts/threads to the query result you still have to step back to the found set of records.
By limiting each to a given date range by date I typically know when I was last on a forum I can step back thru the stream of new content and eventually I will step back to something that i had seen.
Although forums are around to stay i believe the likes of twitter and facebook many people are now too accustomed to the "now" stream and do not necessarily access forum content thru the front end hierarchy of category & forums. but more so by search or by the active stream.