And- did they ask what your salary expectations were when you interviewed? How far off are they? (If yes and they're not far off, but you've found information since taht discussion... you may be slightly more stuck but still not really)

Are you currently employed? This is a strong point of negotiation. You can say to them "I've recieved your offer, and would like to engage in a discussion on salary being offered. While the job sounds intriguing, the risks I take by changing companies are significant, and I do not believe the current salary provides enough incentive to accept those risks." (basically, you're telling them they may have a good number for any Joe off the street, but you're a known quantity at you current place of employment, with a good reputation and have stability... to move you lose all that... and they need to compensate for that risk you accept... )

Also- if you're employed- you can talk about the benefits not matching up- and that you need compensation to match the drop in benefits (whether it's true or not- unless it's a small community where the benefits are generally well known among the competition)

But I know that anything is negotiable if they really want you, and they'll tell you when they reach their limit.