Movies definitely help shape people's perception of history. I forget the name of the book, but I've read social studies education research involving discussions with people about their perceptions of modern American history. Many of the students based their knowledge of events like Vietnam on pop culture (i.e. Forrest Gump).
Some movies are more accurate than others, but all movies are on some level works of fiction. Even the most painstakingly researched movies contain historical inaccuracies (see "Lincoln" and the scene were Connecticut's Representatives vote against the 13th Amendment). Others simply perpetuate stereotypes (see the scholarly debate over "Gone With the Wind" and its depiction of southern society).
The problem isn't so much that these movies blur the lines between truth and fiction, but that people don't bother to dig deeper into the story. I enjoy movies based on historical events, but I usually tend to do some reading afterwards to investigate certain elements of the story. Of course, I'm a history teacher, and I doubt most people do the same.
One final, random note. Last week, a student asked if we could watch "Django Unchained" in class. Cause, you know, that's historically accurate.
I said no.