My list of favorited questions is getting longer, and it would be nice to be able to do a search on favorites. Is that possible ?
It would be useful, but only for those that have many favorites, and that is probably a small number of users. This means that it is lower on the priority list.
It used to exist, but in the beginning of 2011 search was moved from SQL Server to Lucene. Lucene is not a classic SQL database, and not optimized for these kind of operations (joins against another table).
This has also been asked on other sites in the Stack Exchange network, once it gets implemented, all sites will get it.
This doesn't seem to be an option according to advanced search operators.