I'm a
travel agent, and I have to say that Delta is the abosolute worst when it comes to schedule changes. I just dealt with a client today that's had their schedule changed 6 times since they booked it last September, they are to travel in 10 days!
Anyway, with most airlines, usually any change greater than 90 minutes and you can get a full refund if that's what you want (but I recomend you make sure you can find flights on an alternate airline first!). For a big schedule change you can also often have them change the date of travel by one day earlier or later, also with no charge.
If you book your airline ticket with a traditional
travel agency, the airline will notify your agency, your agent will then process the schedule change. They'll contact you, make sure the new times are acceptable, if so they reconfirm your seats and send you the updated info. If they are not acceptable they then can rebook you for a better time, or get your refund and rebook you on another airline. I know a lot of people don't like to use TA's for airline tickets because of the service fees, but in cases like this, it really does save you a lot of work, the agent is the one that has to sit on hold with the airline to get a messy schedule change fixed, not you! (Okay, shameless plug over).
When you use an online travel agency like Orbitz, they are supposed to do the same thing, notify you of the change and help you rebook if needed. From what I've heard, that seems to be hit-and-miss with them, sometimes it works out fine, other times it's a big-ol mess.
The best thing I can recomend is to always call and check on your reservations once or twice between the time you book them and the time you travel (especially if you book Delta). Don't count on the airline to notify you, even the airlines that are really good about contacting passengers sometimes have one that falls through the cracks. And if you find that there is a schedule change, double check your seats, they sometimes get switched around along with the schedule.
Oh, and another tip. When you book airline tickets with anyone (airline, online travel agency, or traditional travel agent) ALWAYS give a phone number where it's easy to reach you!!! I can't count the number of times I've had to play phone-tag with a client because they give me a number they are never at! In fact, give multiple numbers (home, work, cell) if you can. Also give the number of where you'll be at during your trip, sometimes the airline or your agent needs to contact you while you're on your trip.