Today I got a message script tool working along the lines of the one discussed in the ArcGIS Blog for use in generating messages inside ModelBuilder.

First, an example of how it works, set up as a script tool with two text arguments, the first "MESSAGE","ERROR","WARNING", the second your message. What's new about this one over the one linked from the blog is that with this version, you can do error codes by using the format: "id arg1,arg2". (Sure would be nice if a tool was provided that does this in ModelBuilder.)