Moving from SU to MU (or multi-site in 3.0)

How difficult is it to start with a basic WordPress site (or a big single-site WordPress site) and move to multi-site? Is it just flipping a switch (or exporting and importing), or is it a bigger conversion deal?

I'm considering starting a site as a single-site, but it may (probably) will need multiple sites later and while I'd like to keep things simple, if the conversion would be hell, I'd make it multi out the gate.

Thanks!