It just so happens that although the "mu" character is encoded into the Adobe fonts, but is not part of any of the TeX fonts or TeX font encodings of standard Adobe (or TrueType) fonts. However, it is not strictly true that printing the character is impossible. I, being the enterprising character that I am, have found a (somewhat kludgy) workaround:

1. First, you must be using one of the PSNFSS packages available under LaTeX2e, such as "times" (\usepackage{times})
2. Next, create the following definition:
   \def\micro{\special{ps:(µ)show}\nobreak\hspace*{0.55em}}
   (The "mu" character here is the embedded 8-bit character encoding hex 0xb5, or decimal 181, or octal 0625).
3. The \micro definition can be used as follows, for example:
   The correlator measures 700\,{\micro}m $\times$ 1170\,{\micro}m
4. This definition of \micro uses a \special{} function defined only by (as far as I know) dvips, so the proper symbol will only show up after postprocessing the .dvi file through dvips.