This rumor has been floating the tubes for a while but according to AppleInsider, they were tipped off by people "familiar with the matter" that the refresh of the iMacs will happen tomorrow. On top of the Sandy Bridge upgrade they'll receive Thunderbolt just like the new MacBook Pros.
We should see the same GPU that powers the MacBook Pros in the new Sandy Bridge iMacs which is the Radeon HD 6000-series. Screen wise there should be the same models, right up to a 27-inch, 2560x1440 beast. Here's hoping the price comes down with the new specs!