Yes it is true, you are not permitted to short a Nasdaq stock on a down tick so you have to wait for an uptick to do so. Most Level II software packages prevent you from being able to do this so you cannot do it by mistake. It is not generally a major issue as, even in fast moving markets that are going down, an uptick will appear and you can get your short on.
On a separate issue the SEC is being lobbied by US Senators to stop the ability to "Short Sell" stocks on the grounds that it is "Unpatriotic" but in my view it simply wont happen as it is ultimately counter to free trading.