As I mentioned before, I recommend using the Context programming text editor. This editor supports a multiple document interface (so you can simultaneously look at the command file, output, etc), huge file sizes, syntax highlighting for several languages apart from ASReml (e.g., Python, Perl, C++). Once you download and install Context, go to the ‘Highlighters’ folder of the installed program and copy the file Attach:ASReml.chl (for ASReml v1.0) or Attach:ASReml2.chl (for ASReml v2.0, this one kindly provided by Mick Carrick from DPI Victoria, Australia). Just right click the appropriate link to save it to your computer.
Update: ASReml v2 comes with a Context highlighter as part of the standard distribution. The highlighter is based on my first highlighter, later extended by Greg Dutkowski and Arthur Gilmour.
Next time you restart Context and open a .as file, Context will highlight the ASReml code for you. This is very handy when writing long complex jobs. If you are writing a yet unsaved file, just click on Tools | Set Highlighter | ASReml.