In the summer, I ride 22 miles each way 5 days/week. On nice days, I sometimes take the "long" way home and end up with 50-60 total miles for the day. Physically, it's not very challenging anymore as my body is used to this kind of riding, but I think 30+ miles each way would be hard to maintain if you're not already cycling 200+ miles/week. The main thing, as several posters mentioned, is the time involved. I spend about 3 hours/day on my bike, which is big chunk of time on a work day.

akohekohe made a great point about minimizing your commute time. I work in a city that has nightmarish traffic jams every day. Driving to work takes 90 minutes. Biking to work also takes 90 minutes. However, if I drive the fast part and park just outside of town, then take my bike the last 5 miles, I can get to work in under 50 minutes. I rarely do this, though, since we're a one car family and my wife uses the car most days.