This AP article [1] (tech.yahoo.com seems to be having problems just now) says that the overall market fell by either 7.1% or 6.5% worldwide, depending on if you ask IDC or Gartner. US shipments were either down 3.1% (IDC) or <1% (Gartner). Also, the article mentions that Apple's market share increased, but doesn't have an estimate of its shipments.

So without knowing how NPD Group is counting, this 1.8% number is not very useful. If it's global, it makes Apple look pretty good. If it's US shipments, it's harder to say.

[1]: http://74.125.45.132/search?q=cache:tw4QpCy8JFoJ:tech.yahoo....