I've sold three cars on craigslist, and remember there is something on the back of the title (where you fill out the title transfer information) that says you can drive it for 30 days, just need to keep the title in the car to prove transfer of ownership. Of course its easiest for to just go get registered and plates right away.
The new buyer should call their insurance (but most insurers will extend their other car policies for 14 days to the new car, assuming they are already insured), and the seller should cancel theirs in case anything happens in the interim.
There are a bunch of forms here, but really they are all included on the title (odometer, damage disclosure, etc)
http://www.iowadot.gov/mvd/buyingselling/selling.htm
The checklist form (first link) can be handy for both to have a copy of.
IMPORTANT!! Be careful filling out the back of the title. "Scratching out a misprint" is considered an alteration and the County will insist on a new clean one from the previous owner before you can register it (worst case just draw a line through it but keep legible, but they still might reject it). Big pain!!