The InfoPath team has written a set of WebTest plugins that enable testing InfoPath with a WebTest.
See the project on codeplex here: http://www.codeplex.com/ipfswebtest
The project contains custom extraction rules, a sample web test, and a doc on how to use them.
These tests are difficult to script from scratch, so it is great to see this project with the necessary extraction rules already coded and docs on how to use them.
Many thanks to the support of the InfoPath test team on this, especially John Booze and Mary Smith.