Richard's original post called 'Grow Your Own Filled Maps' is getting a bit long so I thought I'd post a few comments here, to encourage you to go there: http://community.tableau.com/thread/116369?start=0&tstart=0 and try his excellent tool Tabgeohack, which allows you to do custom filled maps like this one:
Notice the fairly complex filled shapes that have been created. These do not follow county lines or ZIP codes, they are unique shapes created by my state's Tax & Revenue department. I downloaded a zip file from their site, then I ran it through Richard's Tabgeohack utility, which turns the shape file into a csv file that can then be imported into Tableau as custom geography. Everything works well, once you get use to the 12 or so steps involved. The Tabgeohack zip file comes with a pretty good user manual, a set of sample files, and a template that Richard recently added.
Another good how-to resource is Robert Mundigl's Clearly and Simply blog: http://www.clearlyandsimply.com/clearly_and_simply/2012/03/create-your-own-filled-maps-in-tableau.html He does a good job of spelling out the process step-by-step, and gives some user insights, which can be a little different from a designer's take. Well worth the read.
If you just want to get going quickly here's where you can download tabgeohack: http://dl.dropbox.com/u/59458890/TabGeoHack.zip
But make sure you carefully read the tabgeohack.pdf that comes with it and Robert's blog, or you might find it all a bit daunting (I certainly did at first). I have little experience with databases, or shape files, and my DOS was extremely rusty. But with a bit of help from Richard I was able to get fairly comfortable with the process (after a few weeks working with it off and on). I guess my real message is, if I can do it, anybody can.
So if you need custom filled maps Richard's Tabgeohack is well worth the effort it takes to learn to use it.