Glenn,
The word "Fraud" was used earlier, IMO it's way to strong a word. Fraud charges would most likely only be contemplated if the payee believes that you knowingly wrote a bad check with the intent of obtaining goods and services without paying for them.
A couple of points. First, it's...
I wouldn't necessarily say that a post-dated check is "illegal", if you mean that there is an actual law against it that could result in criminal penalties. Sorry, pet peeve, but I hate it when people confuse "illegal" with "not allowed."
However, a post-dated check is technically invalid...