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.