UPDATE 3: Issue seems to have been resolved.
UPDATE 2: Further investigation reveals that this issue may be related to the AT&T network in the New York and New Jersey area. tracert to citibank.com reveals that the network path from my office to citibank.com goes through the AT&T backbone at some point. This also explains why it does not work from the iPhone. So if you are having this problem you can open up a command prompt and type in tracert citibank.com. Wait for the results and you should see AT&T.NET right around the 8th or 9th hop depending on your ISP.
UPDATE: This seems to be a sporadic issue only affecting some of their customers.
If you are a Citibank customer you may have noticed that their main website citibank.com has been down in the past 24 hours. A Citibank customer service representative confirmed that this has been an ongoing issue over the past two weeks. No specific reason for the outage was given but I was told to use myciti.com until the issue is resolved.
It is not clear how widespread this issue is since a google search does not reveal any news on it. However, I have tried accessing the site from a few different locations with no luck.
Based on the little information that I was given by customer service it is not clear whether they are having internal issues or if it is being attacked by hackers.