The portrait identified as Catherine Parr (c.1512 - 1548), sixth wife of Henry VIII may in fact have been identified wrongly. It bears greater resemblance to an earlier wife, Katherine of Aragon (1485 - 1536), who was the first of Henry's wives and may in fact have been the namesake of the sixth wife.
The portrait features the sitter wearing a Gable Hood. Judging from the style of the Gable worn, it was the style worn in the 1520s, as preferred by Katherine of Aragon. It has been argued that this portrait dates from the time of Catherine Parr's second marriage to Lord Latimer (they married in 1534). By the time of the marriage, the hood as worn by the sitter was already outdated and had progressed to the style worn by Jane Seymour. At the beginning of the 1520s, when this style was popular, Catherine Parr would only have been around 8 years old, and by the end of the decade the French Hood was the preferred headdress of noble ladies.
Secondly, a bigger clue is given in the set of the jaw- the facial characteristics match closely with Katherine of Aragon rather than with Catherine Parr. Even the shape of the nose is different from the nose of Catherine Parr.
Finally, the style of dress worn by the sitter of the portrait was more or less out of date by the time Catherine Parr became Henry's wife in 1543, and the Gable was, to the best of my knowledge, rarely, if ever, worn by Queen Catherine Parr.
Note 28/1/2013: An article published in the Daily Mail on 25/1/2013 has stated that this portrait is in fact Queen Katherine of Aragon.