Everything considered, it never hurts to sign a petition.
Under McCarthyism it was any association with the CPUSA - admittedly a pretty ugly party, but it was both insignificant and went beyond actual membership to even the most tenuous levels of cooperation. As the CPUSA sought to infiltrate and control a whole host of organizations devoted to progressive aims, e.g. anti-fascism and civil rights, it had the intended de facto effect of blacklisting most people active in progressive causes during the thirties and early forties. Here we're talking about working for the secret police under a dictatorship. For elected officials if you did and admit it you're not subject to any sanctions. The new law, however, is a bit vague on what's to happen to, say, civil servants or journalists, or academics who admit to working for the SB.
The online petition, for what it's worth, states the following:
We, European citizens, are deeply concerned by the principles of democracy and human rights and give our full support to Mr. Bronislaw Geremek. The Polish law of lustration, which threatens him of dismissal from his mandate of Member of European Parliament, directly breaks the rules and values to which Poland solemnly adhered, while becoming a Member State of the European Union. We firmly urge for the Polish law of lustration to be repealed. Otherwise, we ask our European governments to consider the application of all the provisions of article 7 of the Treaty.