The NMRP offers general statistics, broken down by year/gender/status and specialty of choice.
http://www.nrmp.org/res_match/tables/table2_04.pdf
The short answer:
- in 2004, 92.9% of MD graduates matched into an alleopathic residency.
- 70.4% of DO graduates matched into an alleopathic residency.
- (for US citizens in foreign medical schools: 55% matched)
EDIT: Should be clear... these percentages are for those who *ranked* alleopathic schools. Meaning in the case of DO schools, only those that didn't match in the AOA match first.
As you can see, its harder, but hardly impossible.