Different approaches to Universal Search.

Universal search is about having an etherogenous set of vertical retrieval systems (Web, Image, Video, News, Books, etc) which are collaborating together for retrieving the right set of etherogeneous results. There are three approaches:

1. Having a set of classifiers, one for each domain that will guess whether that particular domain will have a set of relevant results for the given query. The pros is that you would avoid to send all the queries to all the verticals. The cons is that you will best guess the validity of the results, since you are not sending the real query to the vertical. This is Google pre-2004
2. You have an infrastructure good enough to support Web-like traffic in all the different verticals. This is google post 2004. Note that this is a pull approach
   
3. Another solution, not adopted by Google , is to push set of relevant queries from the vertical to the Query distributor. Consider that each vertical knows, at every given instant of time, the type of queries he can serve with good enough quality.

What solution would you adopt 1, 2, or 3 and when?