AppleInsider is reporting that Apple will release new iMac models next Tuesday, May 3. Citing "people familiar with the matter" who have "continually provided accurate information," AppleInsider states that the next iMacs will sport second-generation Intel Core i5/i7 chips and a Thunderbolt port. It could not be confirmed at this time if the new iMacs will feature changes to the display sizes.
AI also states that its sources said the early morning hours of May 3 at Apple Retail stores will be a "visual night," which means that the new iMacs should be on sale the day they are announced. "Visual night" is the term for when Apple retail employees stay through the night and change merchandising and display planograms in the store in order to prepare a new product for sale. In this case, the older iMacs will be removed and replaced by the newly announced models.
AppleInsider's sources seem to agree with earlier reports from CNET that new iMacs will ship in April or May. The iMac line was last updated on July 27, 2010.