What issues are involved hiring an employee from a competitor?Does the employee have any obligations of confidentiality if no NDA was signed?

Regardless of whether a non-disclosure or non-competition agreement exists, the former employee can't use the trade secrets of his/herformer employer.  As long as he/she does not utilize his/her former employer's trade secrets, absent an agreement to the contrary, a normal employee (as opposed to an employee who originally sold hte business to its new owner) has no obligation not to compete with his former employer.