As altitude increases, the Plasma Television needs to work harder in order to compensate for the difference in external air pressure. As a result, the set will generate more heat and its cooling fans (if it has them) will work harder. This may cause the consumer to hear a "buzzing sound". In addition, the previously mentioned 30,000 to 60,000 hour half-life (depending on brand/model) of the Plasma screen will be reduced somewhat.
For most consumers this is not an issue, however there are considerations if you live in an area over 4,000 feet above sea level. If you do live in a area over 4,000 ft check with your retailer to see whether there might be an issue. Some Plasma TVs are robust enough to work well at altitudes of up to 5000 feet or more (in fact, there high altitude versions of some plasma TVs that can hold up as high as 8,000 ft).
One way to check this out, if you live in a high altitude area, is to check out Plasma TVs at your local dealer. While you are there, put your hand on the unit and compare the warmth from the extra heat generation and listen for the tell-tale buzzing sound. If it turns out that a Plasma Television is not acceptable in your geographical area, you might consider an LCD flat panel or a thin DLP projection set instead. On the positive side of this issue, Plasma TVs specifically calibrated for higher altitude use are now more common.