Will there be any indexing engine incorporated in the future ?

Depending on SQL's 'like' is very time consuming and resource consumptive , especially there are millions of posts in the DB.
Incorporating 3rd party indexing engine , such as Lucene , can greatly simplify the search job.

Apache Lucene also has PHP bindings.

i also would personally love to see ubb.T get away from full-text indexing that myISam offers..

but i'd bet that Lucene would have to be an option as not everyone can just ask their host to install it..

same would go for any other non php/mysql solution..

If Lucene is a little hard , I'd suggest Apache Solr .

Apache Solar is a Lucene's Wrapper , it's like a daemon , which accepts HTTP's POST DATA , and make index of them.
In short , Lucene is programming/API level , while Solr is HTTP level , which accepts any language (just that language able to make HTTP Post).

Why not using Sphinx?

We certainly can't just tie in to an external option by default because many people won't have access to it. However, we can certainly work on putting in some sort of options for it. Sphinx is one that I've been looking at pretty closely.

I think it can be made an "option"
If the indexing server is setup properly , it will start to grab old posts and indexing old/new posts.
If indexing engine is not set , it will fall back to MySQL's LIKE.