Monday theres too much going on. Friday nobody cares because it's the start of the weekend. Tues/Weds gives you time to deal with bugs and problems that crop up after millions of users flood your site after the announcement.
Consider for example making a submission to reddit or digg. When would be a good time to do so to maximize the expected number of people who will check it out?
You probably can still do it on Digg - I went after Reddit because I figured it'd be harder, but I bet Digg is full of XSS and XSRF vulnerabilities.
Of course, press releases are probably the most reliable and effective way.
http://discuss.joelonsoftware.com/default.asp?biz.5.470472.11 "This is from a P.R. pro. Monday is the day that most announcements go out. You still want it to be early in the week but to avoid the crowd, it makes sense to do it on Tuesday."
The odd thing is everybody followed this advice it wouldn't work. So the sluggish Monday idea is pretty compelling :)
With that said, we launched Monday afternoon. Made to digg frontpage Tuesday evening; TechCrunch on Thursday 4-5am EST.
-Zaid
Overthinking generally leads to underperforming.