A fastidious, detail-oriented person. Synonym: t-crosser.
"He really gets focused and pursues whatever he tries to achieve," said partner Wilson, vice president and secretary of Edwards Construction. "He's fastidious. He's an i-dotter and a t-crosser.
"Wyoming developer has roots in Colorado," Northern Colorado Business Report, January 12, 2001
"She had a very lively and a good sense of humor," said Delores Kane, a fellow retired branch librarian. "She liked work done right, but she was not a nit-picker, an i-dotter or a t-crosser."
"Bessie Wachman, 76, Librarian for 50 Years," Chicago Tribune, September 1, 1985
I-dotter comes from the idiom dot your i's and cross your t's, which means to take care of the details. This also explains why the word is most often seen along with the equivalent t-crosser. (Although my research turned up around a dozen instances where i-dotter was used alone.)
This word can be either a compliment or an insult, depending on where your temperament falls in the Felix Unger/Oscar Madison continuum. The citation above shows the compliment side, while the earliest citation shows the insult side.