Apparently, there is a XSS vulnerability in Gmail. It seems pretty serious, and has already affected many people regardless of OS, browser and connection type they use.
http://www.google.com/support/forum/p/g ... 63d8f40cb6
Not much information so far.
The first sign that your account has been hacked — you see many emails in "Sent box" you did not send. + In "account activity" (small link "details" in the bottom of gmail.com page) you will see that someone recently logged into your account from some mobile device.
Possible precaution: do NOT open suspicious emails and do NOT click links ending with .co.cc.