Here it is! It combines her own family Coat of Arms (issued in April 2011) and her husband's.
According to wikipedia:
The Duchess bears the arms of her husband, Prince William, Duke of Cambridge, impaled with her own arms. Catherine's coat of arms is based on those of her father Michael Middleton, which was granted to him by the College of Arms on the 19 April 2011. Thomas Woodcock, Garter King of Arms, the senior officer of the College of Arms, helped the family with the design.
Coronet of a child of the Heir Apparent
Quarterly 1st and 4th Gules three lions passant guardant in pale Or armed and langed Azure (England), 2nd Or a lion rampant Gules armed and langued Azure within a double tressure flory counterflory of the second (Scotland), 3rd Azure a harp Or stringed Argent (Ireland), the whole differenced with a label of three points Argent with the central point charged with an escallop Gules (Prince William); Impaled with a shield per pale Azure and Gules, a chevron Or, cotised Argent, between three acorns slipped and leaved Or (Middleton).
The dividing line (between two colours) down the centre is a canting of the name 'Middle-ton'. The acorns (from the oak tree) are a traditional symbol of England and a feature of west Berkshire, where the family have lived for 30 years. The three acorns also denote the family's three children. The gold chevron in the centre of the arms is an allusion to Carole Middleton's maiden name of Goldsmith. The two white chevronels (narrow chevrons above and below the gold chevron) symbolise peaks and mountains, and the family's love of the Lake District and skiing.
Her previous coat of arms depicted a lozenge shaped shield of arms which hangs from a blue ribbon, this symbolised her unmarried state. This version of the arms is now used only by her sister Pippa as it denotes an unmarried daughter of their father Michael Middleton.