Update 4/21/08. 5:00 pm EDT: Seems it was an isolated outage. Google's press office never responded to a request for comment made this morning.
Update 4/21/08. 9:00 am EDT: Gmail seems to be working fine for some people. I tried the Windows hosts file fix suggested in this thread, and now Gmail works for me. I suspect there is a DNS issue affecting some ISPs but not others.
Gmail has been down for hours now. For those of us who use Gmail for business-related activities, it really sucks. Google apps for domains also seems down. There's a special discussion group, called Gmail down, specifically to allow users to share information about such outages.
Here's a screen capture showing the current state of Gmail: