I've not done extensive research on whether electronic NDAs hold up in court. Maybe I'll pop onto Westlaw and do a search or two on it.
There have been serious questions in the last 15 years regarding the validity of the boilerplate software license agreements, and the results here vary on several factors: 1) which jurisdiction you're in (state to state), 2) whether you get the terms before or after you make your purchase (almost always after), 3) sometimes, the fairness of the terms of the contract itself, 4) the fairness of the process by which the agreement is entered into (has a lot of overlap with #2).
The majority rule though seems to be that electronically "signed" or acknowledged contracts are still binding; there are very few contracts which actually require a physical writing to comply with the Statute of Frauds, and software licenses are not among them (MYLEGS - marriage, contracts that can't possibly completed within a year, sale of land, executor / guarantor contracts, for goods $500+, and suretyships.... gogo first year contracts).
I haven't read the NDA, but it wouldn't surprise me if there were a "liquidated damages" clause in it. Sometimes those work, sometimes they don't.
There are at least two potential barriers to recovery: whether there was any "consideration" to show that there is a binding contract (what did the tester get from the bargain? arguably the tester is donating his time and getting nothing in return. I think the community makes it obvious that the opportunity to play the game early has value, so SE would likely win.) and whether SE actually suffered and can prove damages. If they can show some quantity of damage, like "if this info hadn't leaked, traffic on our website would've been xx% higher and we would've gotten $xx,xxx in ad revenue from increased traffic." That's speculative and likely wouldn't be enough proof, but it's just used to convey the idea.
I imagine a lot of the alpha/beta testers are "judgment-proof" too, with no assets that SE would be able to collect for a judgment.
TLDR version: They'd probably win a lawsuit, but the victory would likely be Pyrrhic. But a full-on ban is much more likely because lawsuits are expensive and wouldn't really do much anyhow. I was just nitpicking on "illegal" vs. "breach of contract."
Edited, Mar 4th 2010 11:01am by JayRams