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I found out a little more about Dramamine... It is a salt of the active ingredient in benadryl (diphenylhydramine) and a mild stimulant (closely related to one of the molecules in coffee, chocolate, and tea).  Diphenylhydramine is an antihistamine; specifically an H1 histamine receptor antagonist.  Common side effects are mild sedation and drowsiness.  It is effective in treating motion sickness related nausea because it blocks certain signals to the nervous system and/or encourages the patient to relax, close their eyes, and sleep.  This is how the brain would like to handle the discrepancy in information it is receiving anyway.

Dramamine is not as effective in treating chemotherapy-induced nausea; there are better anti-emedics, such as Zofran and Kytril.  These drugs are 5-HT3 receptor antagonists blocking serotonin receptors in the central nervous system and gastrointestinal tract.  Other important classes of antiemetics used for treatment of chemotherapy-induced nausea are dopamine antagonists and cannabinoids.  Chemotherapy is thought to cause nausea and vomiting by releasing serotonin, dopamine, and histamine or by damage to stomach cells which also results in a release of serotonin. 

It is interesting to think about why Dramamine is used for motion sickness, but not other types of nausea.  I suspect it relates to a more general trend in medicine.  When nothing else can be done to alleviate the specific discomfort, doctors sedate the patient so that they are less aware of their discomfort.