Amazon offers a white paper on this very topic for their Amazon Web Service, and touts several clients who have proceeded forward.
See the "Interested in HIPAA Compliance?" sidebar here, as well as this whitepaper.
Update: I followed up on the link provided below by Mike Schenk, and found the following statement:
Q. Does AWS GovCloud offer better security than other AWS Regions?
AWS GovCloud offers the same high level of security as other AWS
Regions and supports existing AWS security controls and certifications
such as FISMA, SAS-70, ISO 27001, FIPS 140-2 compliant end points, and
PCI DSS Level 1. AWS also provides an environment that enables
agencies to comply with HIPAA regulations. The only difference is that
AWS has added a layer of permissions to the AWS GovCloud Region that
restricts access to those on an approved list of US Persons.