> what is scalability?

I sympathize with the buzz word comment but it became an issue in my working life.
Try some creative Google queries for more but this is a good start:

http://www.computerworld.com/hardwaretopics/hardware/story/0,10801,46308,00.html

> Is it more than SMP and clustering?

Yes, SMP and clustering are just techniques for adding resources to a system.

Scalability: the work load throughput's response to increased resources.
You can graph it and evaluate visually.

> Can NetBSD be configured in a cluster?

Techniques to implement a "cluster" vary by operating system & application.

In general clustering is loosely coupling multiple systems to achieve
(1) automatic recovery from hardware failures and/or (2) additional work
load throughput and/or (3) work load balancing.  Specific software is
always a key component to implement clustering's goals.

Consider that a NetBSD system running an NFS server containing WWW pages
can be accessed by one or more NetBSD NFS client systems running apache.

> Is Linux more scalable than NetBSD?

Scalability is sensitive to many factors and operating system is just one.

The nature of the work load is critical and the throughput metrics selected
are also key.

In general it depends.  This is just one opinion on Linux vs. BSD:

http://bulk.fefe.de/scalability/

Have fun.