We really hope you didn't bite when "Sega" asked if you still owned a Dreamcast, promising you a snazzy @dreamcast.com e-mail address in return. It might not have been worth it, as the Mainichi Shimbun is reporting that Sega has called the Dreamcast.com web site nothing but a "bogus" domain squatter, saying that whomever is currently using the site is using Sega's trademarks and logos without permission. At best, its just a fan site collecting Dreamcast serial numbers for an exhaustive database, but at worst it may be a phishing scam built on the foundation of nostalgia for September 9, 1999.
While you may already be on the receiving end of some fresh spam, I'd definitely recommend a quick password change to whatever e-mail address you used if you submitted your info.
Sega Calls Attention to Dreamcast Imposter Site [Yahoo Japan]