Is succession planning only for executives only?

Good question Caty. First, succession planning should be a set initiative within every organization, which unfortunately is often times not.

Succession planning can take on many forms depending on the position/s being considered. Historically, succession planning has been only considered for executive-level positions and not for positions that sit outside the C-Suite. IMHO, any position that will have an adverse effect on an organization's productivity and bottom-line, if left vacant for any period of time, should be a position for which a succession plan is in place, regardless of where it falls within the company hierarchy. This is just good business sense.

We get specialized in our work and some knowledge bases seem to belong to only one person in an organization. We should take the time to pass on our own area of expertise to several other individuals so that that knowledge base lives on after we are gone, literally or figuratively.

in fact succession planning should go beyond even the notion of high-performing/ top-mgmt or hi-po...
Talent comes and goes, and what you need to look at is whether all CRITICAL KNOWLEDGE and CRITICAL RESOURCES can be quickly made available / replaced internally. A critical resource may not be a high-performing, but if the task has to be done and requires expertise or knowledge or specific talent, regardless of "performance" than you better have potential successor in the pipeline.
Also succession planning should not be seen as "vertical" position move only: most of times best successor for given positions are found in parallel functions.

You should have an a succession plan/strategic staffing plan for every key role and critical talent in your organization,
Too often we focus on the "C" level only to find a key resource just took our compelling value proposition with them when they left...

I agree with Michael that succession planning should be about the key roles within the company - not just the C-suite. An aside to that is that this planning should not be in isolation of the overall talent management initiative. I've seen (more than once) a person within an organization be slated in a succession plan for a key role but the "plan" did not get effectively communicated to him and he left in an effort to succeed on his own.

I would agree with those above that highlight the need for a company to focus on those roles within the organization whose vacancy would cause a substantive impact to the company. While much is said about C-level positions, I would venture the premise that they are not the most critical (sans the impact of a Steve Jobs or Bill Gates). Most corporations who have a full C-level staff also have qualified VP or director-level staff that could step in to fill a C-level vacancy. This is not necessarily the case with specialized, lower level positions (i.e. engineers or technicians) who are known for their ability to design, engineer, test, or resolve manufacturing issues due to their unique ability. It is not uncommon for companies to be thrown into a revenue panic when an employee of this type vacates their position due to a career change. And I know of very few C-level personnel that could pitch-hit in the design center or on the manufacturing floor to take up the slack. In such cases, cross-training becomes exceptionally important to avoid this type of dilemma in which there is an over-reliance on one or more key “lower level” employees.