jobert: Common mistake. It's used almost interchangeably. Even Steve McConnell, author of Code Complete, calls it assembler.
Assembly is the language, and the assembler is analogous to a compiler for C. Those who refer to development in assembler are, quite simply, wrong.
[edit]I guess I shouldn't say mistake, as it is so widely used, it's a non issue. However, there still is a distinction between the two.[/edit]