Bold Greek letters are available using the \boldsymbol macro, which needs to be enabled by calling \loadbold in the preamble.
\documentstyle{amsppt} \loadbold \document Let $\boldsymbol\alpha$ be a vector. Call it $\boldsymbol\beta$. \enddocument
(Note that this is not to be taken as an example of good mathematical exposition.)
Last updated 23 August 2006