Without margin it does mean you must wait three days to buy another stock with the cash from the sale of a stock. It is based on the historical system of actually exchanging paper certificates. The three days allow for the person to deliver to the recieving broker the actual certificate. If they did not, the trade would be reversed. If you could buy a stock without actually having recieved the cash and then the person you sold the previous stock to failed to deliver then there would be a problem.
In today's system of electronic book entry there is really no need for the three days. While it is technically possible for a failure to deliver I have not heard of anyone having it happen to them in the recent past.
If the trade did get reversed then the shortfall if you did not have a margin account would be made up by the brokerage house capital. This is not something they would be happy about. In Canada this is against IIROC
http://www.iiroc.ca/English/About/Pages/default.aspx rules and is illegal in the US. These rules state that you must recieve the security before you are allowed to sell it unless you have a margin account.
Technically, if you have a shortfall in your account due to decreasing margin during the three days you are not allowed to sell a different security to cover but I would expect that this rule would be overlooked.
D