I have a 19 month old who is experiencing similar behavior, and I can totally understand where you are coming from, but I have to say that I disagree with about 75% of the other response. I don't think it is ok to have him scream for an hour and I think that on a bad day, rocking your child to sleep is perfectly normal!
We had to stop and think and look for symptoms of the problem, not just treat the behavior - it could be teeth, growth spurts, it could have been something that happened at daycare at nap time (i.e. - he felt left out of an activity or attention another child received, or something startled or scared him), it could be slight change in routine, only you will know, but start thinking about things taht might be have occured to cause the disruption.
We discovered that on a particularly windy night here in Colorado a few weeks ago, the window in my daughters room rattled really loudly - which startled her and woke her up - which we believe was the source of the temper tantrum like behavior that continued for almost a week at bedtime.
Instead of forcing her back in her old routine, we started some new ones - brought out some new books, starting a new routine with her stuffed animals,etc...
I also learned taht at this age, kids can start to have night terrors - and there is really very little you can do about it - if you think that might be a problem, you can email me back and I will find an article my mom sent to me about night terrors and send it to you - it was very informative and really helped me feel better about being a parent in these situations.
best of luck!