I have a hard time seeing why "political neutrality" is ever an important principle, but especially in this case, Wikipedia would be acting in what it considers to be it's own self-interest. What merits are there in any entity remaining "neutral" in circumstances like that?
Political neutrality is also considered a good thing for judges, various public officials, and many journalists. If you have a diversity of strong political opinions in your family or workplace, keeping the shared spaces/events/organizations politically neutral is also usually a good idea.
And nothing about "neutrality" suggests that you should not act in defense.
And as bad as SOPA is, there are many worse laws proposed or existing around the world. Should every Wikipedia in countries that already have far worse free-speech laws be constantly blinking out in intermittent protest, once this threshold for 'stop-the-service' activism is set?
" Should every Wikipedia in countries that already have far worse free-speech laws be constantly blinking out in intermittent protest, once this threshold for 'stop-the-service' activism is set?"
Arguably, yes. They should always do this in response to legislation that so directly effects them. Doing it after the fact is obviously of much more limited utility.
"activism"
Again, this is a silly accusation.
If something threatens the basic rules our society is build upon such as the constitution of the country you live in or human rights, as I believe SOPA does, political neutrality stops being worth maintaining, in fact I think it is actively harmful.