I hadn't heard about arnica before and was curious about the product, so, like all good web searches, I started with Wikipedia:
Arnica - Wikipedia, the free encyclopedia
Helenalin - Wikipedia, the free encyclopedia
MicroMedex (online pharmacy resource - subscription required) says for dosing:
--DEGENERATION OF THE JOINTS, RHEUMATIC JOINT AND MUSCLE INFLAMMATION, SACROILIAC PAIN, LUMBAGO, MYOGELOSIS, AND MUSCULAR INDURATION: topical administration of a 2 to 4 centimeters long ointment layer containing an oily extract of Arnica flowers (5%) in the morning and in the evening onto the affected areas
--oral use of Arnica is considered to be potentially unsafe; however, oral preparations (no info on what dose, so likely 10^-30 concentrations - a.a.t.o)
have been used in clinical studies with some success
--recommended for external use only
and for side effects:
--allergic reactions of the skin such as painful itching and inflammation
--cardiac arrest and death with oral use
--edematous dermatitis with the formation of pustules
--high doses: toxic skin reactions with the formation of vesicles and necrosis
--stomach pain, diarrhea, vomiting, and dyspnea
Medicine claimed a homeopathic preperation was ineffective, but didn't give the citation.
DynaMed agreed: "homeopathic arnica cream NOT effective for bruise or minor trauma in review of 4 placebo-controlled trials" BMJ 2001 Sep 29;323(7315):760
But neither of those sites gave info on oral use beyond that it was used in homeopathic preperations and tinctures.
There is toxicology info on topical presentations (to sum up: they don't know) but not oral
Final report on the safety assessment of Arnica mo...[Int J Toxicol. 2001] - PubMed Result
Based on what I've read, I would use it topically but not ingest the stuff. Of course, your mileage may vary...
Studies of reglar pharmaceuticals have problems with bias / motivations / hidden data / etc. and they're peer reviewed. Studies of herbal remedies have more push from the people who stand to make a buck and less peer review - so the data is even worse. Just be sure to tell your doctor about all the herbs / supplements you use - a lot of them react with prescribed meds.