[Update: As of 1 PM Eastern, Google is reporting all problems with Gmail as resolved, and indeed, the banner about Contacts being offline is gone, and sending and receiving mail appears to be working normally. -Adam]
Although Gmail isn't rejecting connections for me, as it did the last time it went down three weeks ago (see "Gmail Web Interface Down; IMAP Working," 2009-09-01), we're seeing distinct problems as of about 10 AM Eastern.
Mail is slow to come in, but more troubling is that Gmail's Web interface is having trouble sending messages. And since sending email is essentially a modal action in Gmail (you can't do anything else in the Web interface while it tries to send), sending problems render Gmail unusable. I was able to save drafts several times, and messages would sometimes send and would sometimes display errors claiming they couldn't be sent.
Gmail is also displaying a banner that says Contacts are offline, which Google explains means that auto-complete may not work, the contact manager may not load, and chat may not work. Apparently, I've become highly dependent on auto-complete when sending messages, to judge from my stuttering when faced with an uncompleted name in my To field.
As with the previous outage, IMAP access to Gmail seems unaffected, and whether or not it would be in any future scenario, keeping an email client like Mail downloading messages from Gmail via IMAP would mean that you'd at least have access to your older messages for reference while Google fixes the online problems.
As I wrote last time:
"The moral of the story - backup, backup, backup. The smart computer user who can't necessarily afford to be offline for a long period of time backs up not just data, but access methods. If Gmail's Web interface goes down, I can use IMAP. If Gmail stops working entirely, I can use my own server or my MobileMe account. If my cable modem goes down, I know where I'll take my MacBook for Internet access. And so on..."