There is nothing wrong with splitting an infinitive, though it is wise not to do so if that construction can be gracefully avoided.  According to Theodore M. Bernstein in The Careful Writer (Atheneum:  1968), eighteenth- and nineteenth-century grammarians frowned on the practice of splitting infinitives, and grammarians have done so ever since.  He concludes:  "For better or worse, the taboo against the split infinitive is a linguistic fact of life, which the writer ignores at his own risk.  Does that mean the risk should never be taken?  By no means."

Consider:  "We expect wages to more than double in the next decade."  Where else can we place more than except within the infinitive?  Here's an example from Bernstein:  "The Thanksgiving Day setback was sure to further defer American hopes of keeping pace with the Soviet Union in lunar exploration."  If we move further from within the infinitive, we have "to defer further American hopes," which at best produces ambiguity and at worst distorts the intended meaning.

Thus, though splitting the infinitive is to be avoided, taking "don't split the infinitive" as an absolute prohibition has its hazards.  One should not hesitate to boldly split the infinitive when no graceful alternative is available.