Windows 7 has brought a lot to the table, not the least of which is improved security architecture that helps make it safer than previous versions. It isn't impenetrable though, and the first zero-day exploit for the new OS was discovered earlier this week.
A flaw buried inside the SMB protocol could cause a system lockup if a user attempts to browse shares on a "malicious" server. While the flaw is unpatched, there are already several methods posted on how to avoid it – such as filtering the ports used by SMB, disabling file sharing, and not browsing shares on remote machines.
Keep in mind, the scope of this flaw is fairly limited. Most firewalls already block the standard ports used by SMB and Windows File Sharing, so users must physically trigger it, which will leave them open to a system lockup at most – as opposed to system compromise. It is probably more of a concern to networks relying on Windows File Sharing, and potential outsiders causing havoc.
Either way, it doesn't do much to detract from the positive news surrounding Windows 7's debut.