I'd suggest you go focused rather than shotgun. Who are your target customers? What other sites are they using? Are you a cross between eBay and Facebook? Who are the biggest bloggers out there on those topics? Target those people, like a laser beam, you should be able to guarantee coverage on launch day.
Plus, you should be able to generate some hype beforehand. Start targetting these sites now. Tell them the launch is coming. Build the relationship. Make sure the authors know who you are and that you're coming.
Oh, and in 4-6 weeks means nothing, pick a date. Pick a date in 8 weeks so you can be sure you hit it, but a date, an actual date, a timeframe is always meaningless.
So you've targetted the top bloggers in the field, now offer them something. Tell them you're coming, then offer the top blogger a 4 hour exclusive if they'll guarantee coverage. Offer them all a sneak preview. Delay your launch by 3 days and run 3 days of "private, invite only beta testing". Tell all the bloggers you're targetting that they can invite people, on their blogs.
The people you're targetting (bloggers, not customers) have to feel like they're getting something real, something that's a scoop, not some generic, third hand, run of the mill announcement.
Second, target forums / user groups / fan sites / etc. Anything in the eBay / MySpace / Facebook / etc space, if there's a forum out there, find their news section, add them to your list above. Then get an account on the forum and start talking (try to keep it relevant, spam never helped anyone).
Scrap the "Coming soon" on your site and start a big, fat counter. LAUNCHING IN 45/44/43 DAYS, etc.
Generate some hype, get people excited, this thing is going to happen, it's going to be huge, and you want to get into it right now.
Most of all, good luck!