forms authentication website using web service to speak to back end

 Hi,
 
I am writing a website (.net 3.5) which uses forms authentication. The business layer and dal are hosted on a seperate server and are accessed by the website code-behind via a web-service (all of which I am also writing). This is not a wcf web-service.

Initially users will register or login which will both require calls to the web-service from unauthorised users. All susbsequent web-service calls will be from authenticated users.

I'm not very knowledgable about security so I wondered if anyone could point out the areas of risk in this configuraiton and any steps that need to be taken. I'd also be grateful for any relevants urls, etc where I can read-up myself.

 
Thanks in advance.

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