*Update 17/2/2010*: GMail recently enabled this HTTPs connection by default, so please read on for the fix!
You've probably just enabled the latest setting in Gmail: Always use HTTPs. (But good news, there's a fix!)
Though this setting is advised (as it is more secure, and less vulnerable to Cross Site Scripting or XSS), it has the downside of disabling your GMail notifier. It will spill out "An error has occured. Cannot connect to your mailbox. Service temporarily unavailable."- alerts instead of notifying you of new messages.
Yet I can't image it would take long for Google to prepare a fix for this.
More information on the "Always use https" can be found in the GMail blog.
*Update*: They've added a quick fix for this issue, where a registry setting is altered so the Notifier retrieves your e-mail from the HTTPs site, instead of the HTTP site. More information, see the comments!