Refusing to potentially incriminate yourself through breath-alcohol tests already leaves you open to license suspension and, as Radley Balko once reported, "Thirty-seven states now impose harsher penalties on motorists who refuse to take roadside sobriety tests than on those who take them and fail." (Hello, Fourth Amendment?) CNN reports that many states are trying to make the punishments for that refusal more severe.
Refusing some of the roadside tests they try to give you prior to the breath test is not yet illegal (or at least not everywhere–I'm not up on every state law on this topic). I wrote of a crafty DUI defense attorney's "Roaside Rights Kit" telling cops to buzz off back in Reason's November 2001 issue. Cops reaction to this standing up for one's rights? Said the attorney: "The most common response from officers is, 'If I saw anybody use this product I'd arrest them.'"