According to numerous reports, including a post at Search Engine Land, if you've advertised on Ask.com between August 2005 and now, you're entitled to reimbursement for any suspected invalid clicks. Those who are entitled to the settlement money will be getting it in the form of advertising (no checks or cash, sorry!)
If you've been affected, your claim must be submitted by February 2, 2008.
A website has been set up for more information about filing the claim. A lot of people find the poor writing style on the site "super fishy," so if it helps, perhaps you'd be interested in reading the legal note that Scott Hendison posted on his blog.
Forum discussion continues at WebmasterWorld.