Last summer I looked into this one extensively for R2, so I know a little bit about what's involved. There are three basic parts to putting together something like Spotlight for Haiku:3. Spotlight-like Indexing Features for Haiku - I must confess I have no idea where or how I'd begin with this one, but it's something I'd definitely love to see this in Haiku. Spotlight is one feature of OS X that I wouldn't give up for a million dollars.
First, the Node Monitor needs another call which just specifies the volume to monitor so that modification to any file on a particular volume triggers a notification. IIRC Linux does it this way, too. It involves a few small tweaks to a couple of system source files, but nothing major.
Second, you'll need to implement the indexer. Most of the work involved in the entire project is done on this. The indexer is like a web spider except that it accesses files directly instead of working over HTTP. Actually storing metadata might be a challenge just because you could store it in attributes on BFS volumes, but we still have to deal with FAT32 and other filesystems which don't have them. IIRC, Tracker -- that is, the indexer included with Ubuntu -- stores metadata in a SQLite database for this. Luckily, there is a working port of SQLite to BeOS. Writing indexing plugins is also needed -- a plugin architecture is typically used for different indexed filetypes to add flexibility.
Lastly, you'll need to write the search interface and API. This isn't all that involved. A simple search program could be written for a proof-of-concept and Tracker integration could be done later on.
Hope this helps. --DarkWyrm