I don't think I would bother with a press release, unless you're launching something huge and you have the capital to back it up (e.g., a scalable system and people ready to go). I was just putting that out there. It sort of goes against the web model, anyway.
Having people give email addresses is one approach, but that realy only serves to give you a way to make a targeted announcement. Good, but not great. Having the site up and running is the crucial step. By the 80-20 rule, the last 20% of the features takes 80% of the time. Getting a product out the door is hard.
Once it's up, people will judge it pretty quickly, I think. They will look at it, judge it cool and come back, or judge it harshly and never come back again. Sometimes you get a second chance. If the product can be rapidly improved and you can maintain some buzz, then those lost users may see a reference to it and give it another try.