Tests aren't valid or invalid per se--it depends on what you use them for.
But if your tests are challenged legally (say, because they have a discriminatory impact against a protected group), one of the things you'll want to defend yourself with is a test validation report--a documentation of why the test was developed, how it was developed, and the purposes for which the test should be used.
This is one of the topics that will be covered in an upcoming webinar sponsored by Talent Management and presented by some well known folks over at APT. Taking place on November 11th at 11am PST, the webinar is titled "Testing the Test: What You Need to Know about Test Validation, Litigation, and Risk Management."
Should be worth a watch/listen.
On a related note, the latest issue of Talent Management had some good articles in it, including ones on "role based" assessment (which just sounds like good 'ol fashioned position-based assessment) and employee surveys. It's actually not a bad little magazine, and it's free. You can subscribe here.