What's the best launch strategy for a web startup?

All the answers seem to be from "early influencers" (Scoble most of all) advancing a view in which they're the gatekeepers. The problem with that is that you end up building products for the particularly narrow niche that is Northern California Tech PR professionals.

I can think of several huge successes that as far as I can tell, were huge long before the Tech PR community discovered them.

Facebook - built an audience at a single colleges first.

Groupon - never heard a thing about them from the PR community, until boom they were everywhere.

Zynga - just pure evil marketing crap, but seemed to work spectacularly well.

So I think that completely ignoring the "early influencer" crowd and doing plain old marketing (find a market, advertises, promote, etc) is more likely to result in huge success than is playing the SF/NYC/SxSW "influencer" game.

It depends massively on the intended market. Is it a consumer service? SMB service? Mobile? Local? Social? Color?

For a consumer service, making a big launch splash on TechCrunch, SXSW, Disrupt, etc is awesome. Not a lot of start-ups are fortunate enough to be a part of any of those events. However, just because you launch at one of these does not mean that the service will prove to be persistently successful. Certainly, you will get a huge bump in traffic off the bat, but if you do not have a great product, then it will be extremely difficult to create sustained value for your users. In my opinion, it would be far better to create a killer product, get your first 1000 users, and pay attention to what they want. Ideally, you get the PR to come to you.

release early. release often.

If you have killer product PR will be hunting for you.