I would suspect that they will ask you for examples of your sites and techniques used and why you used them.
It would also be good to elaborate a bit on your background (business knowledge, web skills, etc).
Remember to be prepared for the dreaded, "do you have any questions for us?" question! :)
Think along the lines of:
"What's the history of the company?"
"What sort of other products or services are offered?"
"What scope is there to expand my existing skills on the job?"
"What are the future plans of the company?" "How can I get involved and contribute to that?"
"How many people work for the company?" "How many people will be on the team I'm working in?"
Like others have said, your best selling point is that you keep up to date with the industry.
Let them know you read up on SEO news and participate in Webmaster World. If they are even slightly on the ball they will appreciate the need to be up to date in the SEO field. If they are totally on the ball they will recognise WW! ;)
Also don't be too nervous at the interview - treat them like you would clients - be polite and courteous and carefully explain what you can do for them.
THEY ARE YOUR CUSTOMERS! THEY ARE PAYING YOU FOR A SERVICE! :)
At the end of the interview be sure to find out when they will let you know if you have the job and how they will contact you. Say something along the lines of, "So, when can i expect a call from you?....".
If they don't call by the time they said they would don't worry. Give it a day or two more then call them back.
Chances are that they have arranged some last minute interviews and have had to delay contacting people. If the worst comes to the worst, and you don't get the job, then be sure to ask for some feedback about the interview.
At least you will know which areas to work on for your next interviews! :)
Good luck! ;)
ps if all else fails, tell them "you have a special relationship with Google!" ;)