If you didn't use a Headhunter/Recruiter to get the job, asking for a signing bonus works better than a higher salary. Justification is that you saved the company $10-20k in recruiting fees, why not give you a $5k signing bonus. If you used a headhunter or recruiter, however, you're in a much tougher position to negotiate even the salary.
http://money.cnn.com/2006/02/14/news/economy/annie/annie0214/index.htm
http://money.cnn.com/2006/02/14/news/economy/annie/annie0214/index.htm
