Is a commercial licence available?
It's perhaps worth noting that the GPL isn't inherently non-commercial, but it doesn't allow you to ship a closed-source application which uses Xapian, which is what people are usually wanting to do when they ask this question.
To a first approximation, the answer is "no". The copyright on Xapian is held by several people and organisations; some of these would probably be willing to relicense their contributions under a different license, but it would be necessary to get agreement from all the copyright holders.
In particular, a reasonable proportion of the code was developed by employees of a company called "BrightStation" which began the development of Xapian (which was then called by a different name), but later pulled out. We don't know who now owns the copyright on the code which was developed by them (there seem to be conflicting claims) but in any case you'd be unlikely to be able to acquire a licence.
Therefore the only way to get a non-GPL license is to get agreement from as many contributors as possible, and then to replace any pieces of code which are covered by a copyright not owned by this group. We have discussed this in the past, and it's gradually happening naturally as the code evolves, but it's not something that's going to happen overnight. If someone was willing to pay for the developer time to make it happen, it is probably a feasible project.
We've been careful to keep all code history, and every source file in Xapian lists the copyright holders for that file which should aid copyright auditing.
The Xapian history web page contains some of the corporate history which is relevant to this.
