Unfortunately, right now you'll be greated with the message "Due the high volume of account requests, the service will be temporally suspended".
There's also some debate going on in the Kotaku comments section that this might be a phishing scam. I'm not entirely sure on that but maybe Sega will come out and say something.
I still use my Dreamcast occasionally, it's a shame it died as quickly as it did. But, there are still plenty of fans out there - I'm curious as to what plans Sega might have for them.