A recent tweet by Brian Tong of CNet implies a May release date for the new line of iMacs.
He states, "My Sources: New iMacs en route by ocean to U.S. available end of April or 1st week of May. No major cosmetic changes."
If the report is to be believed, the new iMacs are expected to run Intel's latest Sandy Bridge processor and feature the latest Thunderbolt I/O interface ports. Thunderbolt is the new interface technology developed by Intel. It is similar to the USB port but promises to deliver 20 times better performance.
For the past couple of years we have not seen a major redesign of any Mac, be it the iMac or the MacBook Pro and the update would be on similar lines. You may not see any cosmetic changes, except for some feature enhancements.
Earlier in the year Apple updated the MacBook Pro line of laptops with Sandy Bridge Processors, upgraded AMD discrete graphics, Thunderbolt I/O and faster RAM.
The iMac line was last updated in July 2010.