Mixing formula into breastmilk is NEVER a good idea. Micronutrients in the formula will become so much more concentrated that it can be very hard on immature kidneys. It makes their nutrition very hard to digest and a new AAP article came out in November of 2009 explaining the energy rates of premies are used up trying to digest the formula more so than breastmilk. Also, mixing of breast milk and formula may decrease lysozyme (an anti-infective factor) activity in human milk. (Quan R et al., Clinical Pediatrics, 1994) It also may negate the immunological properties of lactoferrin.
There are some working on studies that show the formula may even break down some of the proteins and calcium already naturally provided in the breastmilk.
Here is the point- if it aint broke, don't fix it! Some of these mom are producing an abundance of milk for their pre-term infants and are being told it's better for them to have formula. I have yet to have been given a case where it was 100% better to have given the baby formula instead of its mothers' own milk. 

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