We've been learning recently that we should have started building relationships with bloggers, media and potential customers a long time ago. I say "learned recently" but we really knew it all along as it is such oft repeated advice in the startup world.
Unfortunately we kept putting it off because developing the product felt a lot more tangible and progress came easily. The same cannot be said for building meaningful relationships that can be used as a launchpad come launch day.
So, my advice (which echoes so many other voices around HN): start writing about the space you are in. Start building real relationships with bloggers. Start a mailing list to collect email addresses. Start joining forums that are in your domain and build a rep. These things take a while to cultivate and you want to be able to harvest at launch.
As for SEO, get the basics sorted (domain registration, keyword heavy landing page etc), but don't spend too much time on it. The real SEO power comes from getting quality links. These only come when you have those relationships with bloggers etc pumping.
Goodluck with the launch!
-James (Carlo's partner in crime)
(edited for typos)