Merriam-Webster Collegiate Dictionary - split infinitive

 noun  Date: 1897 an infinitive with to having a modifier between the to and the verbal (as in “to really start”) Usage:  The ~ was discovered and named in the 19th century. 19th century writers seem to have made greater use of this construction than earlier writers; the frequency of occurrence attracted the disapproving attention of grammarians, many of whom thought it to be a modern corruption. The construction had in fact been in occasional use since the 14th century; only its frequency had changed. Even though there has never been a rational basis for objecting to the ~, the subject has become a fixture of folk belief about grammar. You can hardly publish a sentence containing one without hearing about it from somebody. Modern commentators know the ~ is not a vice, but they are loath to drop such a popular subject. They usually say it's all right to split an infinitive in the interest of clarity. Since clarity is the usual reason for splitting, this advice means merely that you can split them whenever you need to.