From Wikipedia:
"Some elements of Stewie's personality come from English actor Sir Rex Harrison. MacFarlane has stated that My Fair Lady (starring Harrison) heavily influenced his portrayal of Stewie, and on the Late Show with David Letterman, described Stewie as "Rex Harrison in an infant's body." Family Guy has included several tributes to My Fair Lady, most notably in "One If by Clam, Two If by Sea" and "Running Mates."
MacFarlane has also on more than one occasion linked Stewie with the actor David Hyde Pierce. A running gag is that people seem to believe Pierce does the voice of Stewie (he doesn't), and MacFarlane himself jokingly made that claim in the 100th episode celebration special."
It's sweet!!!