Apple will be announcing and releasing new iMac and Mac mini models Tuesday, March 3rd, according to a report at MyMac.com. John Farr reports that a retail source is claiming to have new models in boxes in his company's warehouses, ready for release on Tuesday.
Unfortunately, Mr. Farr's source has another source, so we'd suggest reading the story with a big bag of salt on hand. Fortunately, as of this writing, Tuesday is only a few hours away.
Apple does traditionally announce new products on Tuesday, though major product revs tend to happen at media events. The next rev of the iMac could well be an evolutionary upgrade, however, and those seldom get media events dedicated to them.