PubMed is a free service of the U.S. National Library of Medicine that includes over 18 million citations from MEDLINE for biomedical articles back to the 1950s.
The service has several web interfaces adapted for mobile devices:
PubMed for handhelds web site
Pubmed Mobile also offers a Disease Associations service which searches case reports to find if the typed symptoms and signs match a particular condition:
Disease Associations with GSpell
The National Library of Medicine also has downloads for mobile devices like Palm and Pocket PC (called Windows Mobile since 2003). There is no iPhone application yet.
Mobile Pubmed: Three ways to look up the literature online. The Palmdoc Chronicles.
Medical sites for mobiles. UBC Wiki.