This NY Times article says a Chicago law firm is representing Mt. Gox customers in a lawsuit against that company. I had a small about of bitcoin in a Mt. Gox account. The law firm has not contacted me to sign me up as a member of the class. Would it be worthwhile to contact them in order to be included?
Normally class actions are opt out, not opt in. That means you don't have to do anything if you want to be in the class. But the decision as to whether it will be what they call "certified" as a class is probably a long way off. If the court allows the class to be certified, they will then send notice to the class members giving you the chance to opt out of the class if you don't want to be in it.
Since you did have an account at Mt. Gox and there is an ongoing class action suit, I would send them an email or give them a call to see what is involved in being included. I am not a lawyer but I imagine that the potential upside of signing up outweighs the potential downsides (risk of countersuit is probably negligible, for example).
The obvious calculation will be:
How much your time is worth * how much time you need to invest in getting involved in the lawsuit vs (the value of your BTC there * the fraction you could reasonably hope to recover) - the fees of the lawyers.
Best of luck; hope you didn't get hit too hard.