When I click that link at the bottom of my gmail screen where it says "Last account activity: 0 minutes ago at IP 50.19.49.49 Details". I click the link and sure enough, it shows accesses from my own IP (IMAP and browser), but it also shows IMAP access from this IP 50.19.49.49, which appears to belong to an Amazon EC2 instance. I can't think of any reason why an EC2 instance would be accessing my email. Should I be concerned?
Update: I changed my password an hour ago and logged back in from my browser and my iPhone, and then went out to dinner. Now it's showing accesses from my home IP (browser and IMAP), and IMAP accesses from 50.19.49.49 (Amazon EC2), 198.227.195.87 ("Service Provider Corporation"), and 166.137.137.85 (AT&T). So either AT&T or Apple is using 50.19.49.49, or some other app on my iPhone got my gmail password and is sharing it with people they shouldn't.
Best Answer
So many services now depend on the EC2 that it can be difficult to pinpoint which one is doing what but it's almost always legitimate when you see this sort of activity from Amazon.
I would normally say this was absolutely nothing to worry about but given the recent (and spectacular) crash of the Amazon cloud, you might consider a password change just to be on the safe side.