I'd figure out how to backup the scribd documents so that your users' data won't get completely lost if and when scribd disappears. You want to make it possible to recover from the loss of scribd, even if it's inconvenient.

Most likely the right answer is to launch now and work around scribd later. If your community completely fails to catch on, then you needn't waste time cloning scribd. If scribd gets you through a year, after which someone else has cloned scribd, you get to leverage their work. If you're really lucky, that work will be open source.

If nothing else, having your community up will allow it to grow and get stuff done while you are busy building an emergency failover system for scribd.

Of course, if your community will lose $1m for every minute that scribd is down, you should consider paying someone to build your own version of scribd. Indeed, you should probably consider buying scribd, or at least their source code.